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1.  Geographic Distribution of Staphylococcus aureus Causing Invasive Infections in Europe: A Molecular-Epidemiological Analysis 
PLoS Medicine  2010;7(1):e1000215.
Hajo Grundmann and colleagues describe the development of a new interactive mapping tool for analyzing the spatial distribution of invasive Staphylococcus aureus clones.
Background
Staphylococcus aureus is one of the most important human pathogens and methicillin-resistant variants (MRSAs) are a major cause of hospital and community-acquired infection. We aimed to map the geographic distribution of the dominant clones that cause invasive infections in Europe.
Methods and Findings
In each country, staphylococcal reference laboratories secured the participation of a sufficient number of hospital laboratories to achieve national geo-demographic representation. Participating laboratories collected successive methicillin-susceptible (MSSA) and MRSA isolates from patients with invasive S. aureus infection using an agreed protocol. All isolates were sent to the respective national reference laboratories and characterised by quality-controlled sequence typing of the variable region of the staphylococcal spa gene (spa typing), and data were uploaded to a central database. Relevant genetic and phenotypic information was assembled for interactive interrogation by a purpose-built Web-based mapping application. Between September 2006 and February 2007, 357 laboratories serving 450 hospitals in 26 countries collected 2,890 MSSA and MRSA isolates from patients with invasive S. aureus infection. A wide geographical distribution of spa types was found with some prevalent in all European countries. MSSA were more diverse than MRSA. Genetic diversity of MRSA differed considerably between countries with dominant MRSA spa types forming distinctive geographical clusters. We provide evidence that a network approach consisting of decentralised typing and visualisation of aggregated data using an interactive mapping tool can provide important information on the dynamics of MRSA populations such as early signalling of emerging strains, cross border spread, and importation by travel.
Conclusions
In contrast to MSSA, MRSA spa types have a predominantly regional distribution in Europe. This finding is indicative of the selection and spread of a limited number of clones within health care networks, suggesting that control efforts aimed at interrupting the spread within and between health care institutions may not only be feasible but ultimately successful and should therefore be strongly encouraged.
Please see later in the article for the Editors' Summary
Editors' Summary
Background
The bacterium Staphylococcus aureus lives on the skin and in the nose of about a third of healthy people. Although S. aureus usually coexists peacefully with its human carriers, it is also an important disease-causing organism or pathogen. If it enters the body through a cut or during a surgical procedure, S. aureus can cause minor infections such as pimples and boils or more serious, life-threatening infections such as blood poisoning and pneumonia. Minor S. aureus infections can be treated without antibiotics—by draining a boil, for example. Invasive infections are usually treated with antibiotics. Unfortunately, many of the S. aureus clones (groups of bacteria that are all genetically related and descended from a single, common ancestor) that are now circulating are resistant to methicillin and several other antibiotics. Invasive methicillin-resistant S. aureus (MRSA) infections are a particular problem in hospitals and other health care facilities (so-called hospital-acquired MRSA infections), but they can also occur in otherwise healthy people who have not been admitted to a hospital (community-acquired MRSA infections).
Why Was This Study Done?
The severity and outcome of an S. aureus infection in an individual depends in part on the ability of the bacterial clone with which the individual is infected to cause disease—the clone's “virulence.” Public-health officials and infectious disease experts would like to know the geographic distribution of the virulent S. aureus clones that cause invasive infections, because this information should help them understand how these pathogens spread and thus how to control them. Different clones of S. aureus can be distinguished by “molecular typing,” the determination of clone-specific sequences of nucleotides in variable regions of the bacterial genome (the bacterium's blueprint; genomes consist of DNA, long chains of nucleotides). In this study, the researchers use molecular typing to map the geographic distribution of MRSA and methicillin-sensitive S. aureus (MSSA) clones causing invasive infections in Europe; a MRSA clone emerges when an MSSA clone acquires antibiotic resistance from another type of bacteria so it is useful to understand the geographic distribution of both MRSA and MSSA.
What Did the Researchers Do and Find?
Between September 2006 and February 2007, 357 laboratories serving 450 hospitals in 26 European countries collected almost 3,000 MRSA and MSSA isolates from patients with invasive S. aureus infections. The isolates were sent to the relevant national staphylococcal reference laboratory (SRL) where they were characterized by quality-controlled sequence typing of the variable region of a staphylococcal gene called spa (spa typing). The spa typing data were entered into a central database and then analyzed by a public, purpose-built Web-based mapping tool (SRL-Maps), which provides interactive access and easy-to-understand illustrations of the geographical distribution of S. aureus clones. Using this mapping tool, the researchers found that there was a wide geographical distribution of spa types across Europe with some types being common in all European countries. MSSA isolates were more diverse than MRSA isolates and the genetic diversity (variability) of MRSA differed considerably between countries. Most importantly, major MRSA spa types occurred in distinct geographical clusters.
What Do These Findings Mean?
These findings provide the first representative snapshot of the genetic population structure of S. aureus across Europe. Because the researchers used spa typing, which analyzes only a small region of one gene, and characterized only 3,000 isolates, analysis of other parts of the S. aureus genome in more isolates is now needed to build a complete portrait of the geographical abundance of the S. aureus clones that cause invasive infections in Europe. However, the finding that MRSA spa types occur mainly in geographical clusters has important implications for the control of MRSA, because it indicates that a limited number of clones are spreading within health care networks, which means that MRSA is mainly spread by patients who are repeatedly admitted to different hospitals. Control efforts aimed at interrupting this spread within and between health care institutions may be feasible and ultimately successful, suggest the researchers, and should be strongly encouraged. In addition, this study shows how, by sharing typing results on a Web-based platform, an international surveillance network can provide clinicians and infection control teams with crucial information about the dynamics of pathogens such as S. aureus, including early warnings about emerging virulent clones.
Additional Information
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000215.
This study is further discussed in a PLoS Medicine Perspective by Franklin D. Lowy
The UK Health Protection Agency provides information about Staphylococcus aureus
The UK National Health Service Choices Web site has pages on staphylococcal infections and on MRSA
The US National Institute of Allergy and Infectious Disease has information about MRSA
The US Centers for Disease Control and Infection provides information about MRSA for the public and professionals
MedlinePlus provides links to further resources on staphylococcal infections and on MRSA (in English and Spanish)
SRL-Maps can be freely accessed
doi:10.1371/journal.pmed.1000215
PMCID: PMC2796391  PMID: 20084094
2.  Identification of a Highly Transmissible Animal-Independent Staphylococcus aureus ST398 Clone with Distinct Genomic and Cell Adhesion Properties 
mBio  2012;3(2):e00027-12.
ABSTRACT
A methicillin-resistant Staphylococcus aureus (MRSA) clone known as ST398 has emerged as a major cause of acute infections in individuals who have close contact with livestock. More recently, the emergence of an animal-independent ST398 methicillin-sensitive S. aureus (MSSA) clone has been documented in several countries. However, the limited surveillance of MSSA has precluded an accurate assessment of the global spread of ST398 and its clinical relevance. Here we provide evidence that ST398 is a frequent source of MSSA infections in northern Manhattan and is readily transmitted between individuals in households. This contrasts with the limited transmissibility of livestock-associated ST398 (LA-ST398) MRSA strains between humans. Our whole-genome sequence analysis revealed that the chromosome of the human-associated ST398 MSSA clone is smaller than that of the LA-ST398 MRSA reference strain S0385, due mainly to fewer mobile genetic elements (MGEs). In contrast, human ST398 MSSA isolates harbored the prophage φ3 and the human-specific immune evasion cluster (IEC) genes chp and scn. While most of the core genome was conserved between the human ST398 MSSA clone and S0385, these strains differed substantially in their repertoire and composition of intact adhesion genes. These genetic changes were associated with significantly enhanced adhesion of human ST398 MSSA isolates to human skin keratinocytes and keratin. We propose that the human ST398 MSSA clone can spread independent of animal contact using an optimized repertoire of MGEs and adhesion molecules adapted to transmission among humans.
IMPORTANCE
Staphylococcus aureus strains have generally been considered to be species specific. However, cross-species transfers of S. aureus clones, such as ST398 methicillin-resistant S. aureus (MRSA), from swine to humans have been reported. Recently, we observed the emergence of ST398 methicillin-susceptible S. aureus (MSSA) as a colonizing strain of humans in northern Manhattan. Here we report that ST398 is a frequent cause of MSSA infections in this urban setting. The ST398 MSSA clone was readily transmitted within households, independent of animal contact. We discovered that human ST398 MSSA genomes were smaller than that of the LA-ST398 strain S0385 due to fewer mobile genetic elements. Human and LA-ST398 strains also differed in their composition of adhesion genes and their ability to bind to human skin keratinocytes, providing a potential mechanism of S. aureus host adaptation. Our findings illustrate the importance of implementing molecular surveillance of MSSA given the evidence for the rapid and clinically undetected spread of ST398 MSSA.
doi:10.1128/mBio.00027-12
PMCID: PMC3302565  PMID: 22375071
3.  Hospital-Community Interactions Foster Coexistence between Methicillin-Resistant Strains of Staphylococcus aureus 
PLoS Pathogens  2013;9(2):e1003134.
Methicillin-resistant Staphylococcus aureus (MRSA) is an important cause of morbidity and mortality in both hospitals and the community. Traditionally, MRSA was mainly hospital-associated (HA-MRSA), but in the past decade community-associated strains (CA-MRSA) have spread widely. CA-MRSA strains seem to have significantly lower biological costs of resistance, and hence it has been speculated that they may replace HA-MRSA strains in the hospital. Such a replacement could potentially have major consequences for public health, as there are differences in the resistance spectra of the two strains as well as possible differences in their clinical effects. Here we assess the impact of competition between HA- and CA-MRSA using epidemiological models which integrate realistic data on drug-usage frequencies, resistance profiles, contact, and age structures. By explicitly accounting for the differing antibiotic usage frequencies in the hospital and the community, we find that coexistence between the strains is a possible outcome, as selection favors CA-MRSA in the community, because of its lower cost of resistance, while it favors HA-MRSA in the hospital, because of its broader resistance spectrum. Incorporating realistic degrees of age- and treatment-structure into the model significantly increases the parameter ranges over which coexistence is possible. Thus, our results indicate that the large heterogeneities existing in human populations make coexistence between hospital- and community-associated strains of MRSA a likely outcome.
Author Summary
One of the most notorious cases of antibiotic-resistant bacteria is methicillin-resistant Staphylococcus aureus (MRSA), which causes diseases ranging from skin and soft-tissue infections to pneumonia and septicemia. Traditionally, MRSA was mainly hospital-associated, but in the past decade community-associated strains have spread widely. Typically drug-resistant bacteria have lower reproduction or transmission rates, called a fitness cost. Because this cost is estimated to be significantly lower for community-associated strains, it has been predicted that these will eventually replace the hospital-associated strains. However, hospital-associated strains are resistant against a greater variety of antibiotics, which may compensate for the higher fitness cost. Here, we integrate realistic data on drug-usage, resistance profiles, contact, and age structures into a mathematical model of MRSA transmission to predict the competition between hospital- and community-associated strains. We find that for a realistic degree of population structure it is likely that both strains of MRSA will coexist in the long term. This results from significantly different hospitalization and antibiotic consumption rates between age groups. In particular, elderly individuals have much higher rates of antibiotic usage and hospitalizations than other age groups. This generates a situation where community-associated strains can predominate in the community but are outcompeted in the hospital, resulting in coexistence in the population.
doi:10.1371/journal.ppat.1003134
PMCID: PMC3585153  PMID: 23468619
4.  Key Role for Clumping Factor B in Staphylococcus aureus Nasal Colonization of Humans 
PLoS Medicine  2008;5(1):e17.
Background
Staphylococcus aureus permanently colonizes the vestibulum nasi of one-fifth of the human population, which is a risk factor for autoinfection. The precise mechanisms whereby S. aureus colonizes the nose are still unknown. The staphylococcal cell-wall protein clumping factor B (ClfB) promotes adhesion to squamous epithelial cells in vitro and might be a physiologically relevant colonization factor.
Methods and Findings
We define the role of the staphylococcal cytokeratin-binding protein ClfB in the colonization process by artificial inoculation of human volunteers with a wild-type strain and its single locus ClfB knock-out mutant. The wild-type strain adhered to immobilized recombinant human cytokeratin 10 (CK10) in a dose-dependent manner, whereas the ClfB− mutant did not. The wild-type strain, when grown to the stationary phase in a poor growth medium, adhered better to CK10, than when the same strain was grown in a nutrient-rich environment. Nasal cultures show that the mutant strain is eliminated from the nares significantly faster than the wild-type strain, with a median of 3 ± 1 d versus 7 ± 4 d (p = 0.006). Furthermore, the wild-type strain was still present in the nares of 3/16 volunteers at the end of follow-up, and the mutant strain was not.
Conclusions
The human colonization model, in combination with in vitro data, shows that the ClfB protein is a major determinant of nasal-persistent S. aureus carriage and is a candidate target molecule for decolonization strategies.
Heiman Wertheim and colleagues investigate the role ofStaphylococcus aureus clumping factor B, a cell wall protein, in bacterial adherence to epithelial cells and persistent colonization of human nostrils.
Editors' Summary
Background.
Staphylococcus aureus are common bacteria that normally live on the skin. They also colonize the nostrils of about one in five adults permanently and another one in three adults intermittently. Although these bacteria usually coexist peacefully with their human carriers, they can cause minor infections such as pimples and boils if they enter the skin through a cut or a sore. They can also cause potentially life-threatening infections such as blood poisoning and pneumonia. These serious, invasive infections are often “autoinfections.” That is, they are caused by strains of S. aureus that are present in the patient's nose before they become ill. Minor S. aureus infections can be treated without antibiotics—by draining a boil, for example. Invasive infections are usually treated with antibiotics such as flucloxacillin.
Why Was This Study Done?
There is no effective vaccine against S. aureus infections and these bacteria are becoming increasingly resistant to flucloxacillin, methicillin, and other antibiotics. Worryingly, although methicillin-resistant S. aureus (MRSA) infections occur most frequently among people in health-care facilities who have weakened immune systems, community-acquired MRSA infections among otherwise healthy people are increasingly common. Consequently, new ways to avoid S. aureus infections are urgently needed. Because persistent nasal carriers of S. aureus have an increased risk of infection, one strategy might be to prevent nasal colonization with S. aureus. How these bacteria colonize the nose is poorly understood, but is likely to involve interactions between molecules expressed on the surface of the bacteria and molecules expressed on the surface of the cells lining the nostrils. In this study, the researchers use a new human nasal colonization assay to investigate the involvement of a bacterial surface protein called clumping factor B (ClfB) in the survival of S. aureus in the human nose. ClfB binds to cytokeratin 10, a protein expressed by cells lining the human nose, and has been implicated in the colonization of mouse noses by S. aureus.
What Did the Researchers Do and Find?
The researchers introduced a strain of S. aureus that made ClfB and an otherwise identical, mutant strain that lacked ClfB into the nostrils of healthy human volunteers and measured how long the two strains survived. For safety reasons, the S. aureus strains used in this study have an additional defect that makes them less likely to colonize and persist in the human nose than the strains found in natural S. aureus carriers. Although both strains grew equally well in the laboratory, the mutant strain was eliminated from human noses much quicker than the strain that made ClfB. Mutant bacteria lacking ClfB were cleared from the nostrils of all the volunteers within two weeks, whereas the bacteria that made ClfB were still present in some of the volunteers four weeks after their introduction. When the researchers investigated how well the two strains stuck to a layer of human cytokeratin 10 in a plastic dish, they found that the bacteria that made ClfB stuck to the human protein but the mutant bacteria did not. Furthermore, the strain with ClfB stuck particularly well to cytokeratin 10 when the bacteria had been grown in conditions where nutrients were limiting, a situation that mimics bacterial growth in the human nose.
What Do These Findings Mean?
These findings show that ClfB is an important factor in the establishment of human nasal colonization by S. aureus and suggest that ClfB might be a target for S. aureus decolonization strategies. Furthermore, although ClfB is clearly important in human nasal colonization by S. aureus, it is likely that additional bacterial factors will also be involved in this process. The human nasal colonization model used in this study may be useful in the identification of these additional factors and also as a test bed for potential S. aureus decolonization strategies.
Additional Information.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050017.
The MedlinePlus encyclopedia has a page on Staphylococcus aureus and MRSA (in English and Spanish)
The US Centers for Disease Control and Prevention provides information on community-associated MRSA (in English and Spanish)
The UK National Health Service's health website (NHS Direct) provides information about staphylococcal infections and about MRSA
The UK Health Protection Agency provides information about Staphylococcus aureus
doi:10.1371/journal.pmed.0050017
PMCID: PMC2194749  PMID: 18198942
5.  Mortality and Hospital Stay Associated with Resistant Staphylococcus aureus and Escherichia coli Bacteremia: Estimating the Burden of Antibiotic Resistance in Europe 
PLoS Medicine  2011;8(10):e1001104.
The authors calculate excess mortality, excess hospital stay, and related hospital expenditure associated with antibiotic-resistant bacterial bloodstream infections (Staphylococcus aureus and Escherichia coli) in Europe.
Background
The relative importance of human diseases is conventionally assessed by cause-specific mortality, morbidity, and economic impact. Current estimates for infections caused by antibiotic-resistant bacteria are not sufficiently supported by quantitative empirical data. This study determined the excess number of deaths, bed-days, and hospital costs associated with blood stream infections (BSIs) caused by methicillin-resistant Staphylococcus aureus (MRSA) and third-generation cephalosporin-resistant Escherichia coli (G3CREC) in 31 countries that participated in the European Antimicrobial Resistance Surveillance System (EARSS).
Methods and Findings
The number of BSIs caused by MRSA and G3CREC was extrapolated from EARSS prevalence data and national health care statistics. Prospective cohort studies, carried out in hospitals participating in EARSS in 2007, provided the parameters for estimating the excess 30-d mortality and hospital stay associated with BSIs caused by either MRSA or G3CREC. Hospital expenditure was derived from a publicly available cost model. Trends established by EARSS were used to determine the trajectories for MRSA and G3CREC prevalence until 2015. In 2007, 27,711 episodes of MRSA BSIs were associated with 5,503 excess deaths and 255,683 excess hospital days in the participating countries, whereas 15,183 episodes of G3CREC BSIs were associated with 2,712 excess deaths and 120,065 extra hospital days. The total costs attributable to excess hospital stays for MRSA and G3CREC BSIs were 44.0 and 18.1 million Euros (63.1 and 29.7 million international dollars), respectively. Based on prevailing trends, the number of BSIs caused by G3CREC is likely to rapidly increase, outnumbering the number of MRSA BSIs in the near future.
Conclusions
Excess mortality associated with BSIs caused by MRSA and G3CREC is significant, and the prolongation of hospital stay imposes a considerable burden on health care systems. A foreseeable shift in the burden of antibiotic resistance from Gram-positive to Gram-negative infections will exacerbate this situation and is reason for concern.
Please see later in the article for the Editors' Summary
Editors' Summary
Background
Antimicrobial resistance—a consequence of the use and misuse of antimicrobial medicines—occurs when a microorganism becomes resistant (usually by mutation or acquiring a resistance gene) to an antimicrobial drug to which it was previously sensitive. Then standard treatments become ineffective, leading to persistent infections, which may spread to other people. With some notable exceptions such as TB, HIV, malaria, and gonorrhea, most of the disease burden attributable to antimicrobial resistance is caused by hospital-associated infections due to opportunistic bacterial pathogens. These bacteria often cause life-threatening or difficult-to-manage conditions such as deep tissue, wound, or bone infections, or infections of the lower respiratory tract, central nervous system, or blood stream. The two most frequent causes of blood stream infections encountered worldwide are Staphylococcus aureus and Escherichia coli.
Why Was This Study Done?
Although hospital-associated infections have gained much attention over the past decade, the overall effect of this growing phenomenon on human health and medical services has still to be adequately quantified. The researchers proposed to fill this information gap by estimating the impact—morbidity, mortality, and demands on health care services—of antibiotic resistance in Europe for two types of resistant organisms that are typically associated with resistance to multiple classes of antibiotics and can be regarded as surrogate markers for multi-drug resistance—methicillin-resistant S. aureus and third-generation cephalosporin-resistant E. coli.
What Did the Researchers Do and Find?
Recently, the Burden of Resistance and Disease in European Nations project collected representative data on the clinical impact of antimicrobial resistance throughout Europe. Using and combining this information with 2007 prevalence data from the European Antibiotic Resistance Surveillance System, the researchers calculated the burden of disease associated with methicillin-resistant S. aureus and third-generation cephalosporin-resistant E. coli blood stream infections. This burden of disease was expressed as excess number of deaths, excess number of days in hospital, and excess costs. Using statistical models, the researchers predicted trend-based resistance trajectories up to 2015 for the 31 participating countries in the European region.
The researchers included 1,293 hospitals from the 31 countries, typically covering 47% of all available acute care hospital beds in most countries, in their analysis. For S. aureus, the estimated number of blood stream infections totaled 108,434, of which 27,711 (25.6%) were methicillin-resistant. E. coli caused 163,476 blood stream infections, of which 15,183 (9.3%) were resistant to third-generation cephalosporins. An estimated 5,503 excess deaths were associated with blood stream infections caused by methicillin-resistant S. aureus (with the UK and France predicted to experience the highest excess mortality), and 2,712 excess deaths with blood stream infections caused by third-generation cephalosporin-resistant E. coli (predicted to be the highest in Turkey and the UK). The researchers also found that blood stream infections caused by both methicillin-resistant S. aureus and third-generation cephalosporin-resistant E. coli contributed respective excesses of 255,683 and 120,065 extra bed-days, accounting for an estimated extra cost of 62.0 million Euros (92.8 million international dollars). In their trend analysis, the researchers found that 97,000 resistant blood stream infections and 17,000 associated deaths could be expected in 2015, along with increases in the lengths of hospital stays and costs. Importantly, the researchers estimated that in the near future, the burden of disease associated with third-generation cephalosporin-resistant E. coli is likely to surpass that associated with methicillin-resistant S. aureus.
What Do These Findings Mean?
These findings show that even though the blood stream infections studied represent only a fraction of the total burden of disease associated with antibiotic resistance, excess mortality associated with these infections caused by methicillin-resistant S. aureus and third-generation cephalosporin-resistant E. coli is high, and the associated prolonged length of stays in hospital imposes a considerable burden on health care systems in Europe. Importantly, a possible shift in the burden of antibiotic resistance from Gram-positive to Gram-negative infections is concerning. Such forecasts suggest that despite anticipated gains in the control of methicillin-resistant S. aureus, the increasing number of infections caused by third-generation cephalosporin-resistant Gram-negative pathogens, such as E. coli, is likely to outweigh this achievement soon. This increasing burden will have a big impact on already stretched health systems.
Additional Information
Please access these websites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001104.
The World Health Organization has a fact sheet on general antimicrobial resistance
The US Centers for Disease Control and Prevention webpage on antibiotic/antimicrobial resistance includes information on educational campaigns and resources
The European Centre for Disease Control provides data about the prevalence of resistance in Europe through an interactive database
doi:10.1371/journal.pmed.1001104
PMCID: PMC3191157  PMID: 22022233
6.  Carriage of Methicillin-Resistant Staphylococcus aureus by Wild Urban Norway Rats (Rattus norvegicus) 
PLoS ONE  2014;9(2):e87983.
Methicillin-resistant Staphylococcus aureus (MRSA) is an important cause of multi-drug-resistant infections in people, particularly indigent populations. MRSA can be transmitted between people and domestic animals, but the potential for transmission between people and commensal pests, particularly rodents, had not been investigated. The objective of this study was to identify the presence and characterize the ecology of MRSA in rats (Rattus spp.) from in an impoverished, inner-city neighborhood. Oropharyngeal swabs were collected from rats trapped in 33 city blocks and one location within the adjacent port. Bacterial culture was performed and MRSA isolates were characterized using a variety of methods, including whole-genome sequencing (WGS). The ecology of MRSA in rats was described using phylogenetic analysis, geospatial analysis, and generalized linear mixed models. MRSA was identified 22 of 637 (3.5%) rats tested, although prevalence varied from 0 – 50% among blocks. Isolates belonged to 4 clusters according to WGS, with the largest cluster (n = 10) containing isolates that were genetically indistinguishable from community-acquired USA300 MRSA strains isolated from people within the study area. MRSA strains demonstrated both geographic clustering and dispersion. The odds of an individual rat carrying MRSA increased with increased body fat (OR = 2.53, 95% CI = 1.33 – 4.82), and in the winter (OR = 5.29, 95% CI = 1.04 – 26.85) and spring (OR = 5.50, 95% CI = 1.10 – 27.58) compared to the fall. The results show that urban rats carried the same MRSA lineages occurring in local human and/or animal populations, supporting recent transmission from external sources. MRSA carriage was influenced by season, most likely as a result of temporal variation in rat behavior and rat-human interactions.
doi:10.1371/journal.pone.0087983
PMCID: PMC3912160  PMID: 24498421
7.  Transmission of Methicillin-Resistant Staphylococcus aureus to Household Contacts▿  
Journal of Clinical Microbiology  2009;48(1):202-207.
The frequency of and risk factors for methicillin-resistant Staphylococcus aureus (MRSA) transmission from a MRSA index person to household contacts were assessed in this prospective study. Between January 2005 and December 2007, 62 newly diagnosed MRSA index persons (46 patients and 16 health care workers) and their 160 household contacts were included in the study analysis. Transmission of MRSA from an index person to household contacts occurred in nearly half of the cases (47%; n = 29). These 29 index persons together had 84 household contacts, of which two-thirds (67%; n = 56) became MRSA positive. Prolonged exposure time to MRSA at home was a significant risk factor for MRSA transmission to household contacts. In addition, MRSA colonization at least in the throat, younger age, and eczema in index persons were significantly associated with MRSA transmission; the presence of wounds was negatively associated with MRSA transmission. Furthermore, an increased number of household contacts and being the partner of a MRSA index person were household-related risk factors for MRSA acquisition from the index person. No predominant pulsed-field gel electrophoresis (PFGE) type was observed to be transmitted more frequently than other PFGE types. To date, screening household contacts and providing MRSA eradication therapy to those found positive simultaneously with the index person is not included in the “search-and-destroy” policy. We suggest including both in MRSA prevention guidelines, as this may reduce further spread of MRSA.
doi:10.1128/JCM.01499-09
PMCID: PMC2812253  PMID: 19923490
8.  Environmental Methicillin-Resistant Staphylococcus aureus in a Veterinary Teaching Hospital During a Nonoutbreak Period 
Abstract
Concurrent to reports of zoonotic and nosocomial transmission of methicillin-resistant Staphylococcus aureus (MRSA) in veterinary settings, recent evidence indicates that the environment in veterinary hospitals may be a potential source of MRSA. The present report is a cross-sectional study to determine the prevalence of MRSA on specific human and animal contact surfaces at a large veterinary hospital during a nonoutbreak period. A total of 156 samples were collected using Swiffers® or premoistened swabs from the small animal, equine, and food animal sections. MRSA was isolated and identified by pre-enrichment culture and standard microbiology procedures, including growth on Mueller-Hinton agar supplemented with NaCl and oxacillin, and by detection of the mecA gene. Staphylococcal chromosome cassette mec (SCCmec) typing and pulsed-field gel electrophoresis profile were also determined. MRSA was detected in 12% (19/157) of the hospital environments sampled. The prevalence of MRSA in the small animal, equine, and food animal areas were 16%, 4%, and 0%, respectively. Sixteen of the MRSA isolates from the small animal section were classified as USA100, SCCmec type II, two of which had pulsed-field gel electrophoresis pattern that does not conform to any known type. The one isolate obtained from the equine section was classified as USA500, SCCmec type IV. The molecular epidemiological analysis revealed a very diverse population of MRSA isolates circulating in the hospital; however, in some instances, multiple locations/surfaces, not directly associated, had the same MRSA clone. No significant difference was observed between animal and human contact surfaces in regard to prevalence and type of isolates. Surfaces touched by multiple people (doors) and patients (carts) were frequently contaminated with MRSA. The results from this study indicate that MRSA is present in the environment even during nonoutbreak periods. This study also identified specific surfaces in a veterinary environment that need to be targeted when designing and executing infection control programs.
doi:10.1089/vbz.2010.0181
PMCID: PMC3391706  PMID: 21417926
environment; MRSA; nosocomial; veterinary hospital
9.  Epidemiology and molecular characterization of methicillin-resistant Staphylococcus aureus nasal carriage isolates from bovines 
BMC Veterinary Research  2014;10:153.
Background
Staphylococcus aureus is a common bacterium usually found on skin and mucous membranes of warm blooded animals. Resistance in S. aureus has been increasingly reported though depending on the clonal lineage. Indeed, while hospital acquired (HA)-methicillin resistant S. aureus (MRSA) are typically multi-resistant, community associated (CA)-MRSA are by large more susceptible to many antibiotics. Although S. aureus isolated from animals are often susceptible to most antibiotics, multi-resistant livestock associated (LA)-MRSA have been recovered from bovine mastitis.
In this study, we investigated the prevalence and types of MRSA present in the nose of healthy bovines of different age groups and rearing practices. Since no validated methods for MRSA isolation from nasal swabs were available, we compared two isolation methods. Molecular characterization was performed by means of spa-typing, MLST, SCCmec typing and microarray analysis for the detection of antimicrobial resistance and virulence genes.
Results
MRSA between herd prevalence in bovines was estimated at 19.8%. There was a marked difference between rearing practices with 9.9%, 10.2% and 46.1% of the dairy, beef and veal calve farms respectively being MRSA positive. No significant difference was observed between both isolation methods tested. Most isolates were ST398 spa type t011 or closely related spa types. Few ST239 spa type t037 and t388 and ST8 spa type t121 were also found. SCCmec types carried by these strains were mainly type IV(2B), IV(2B&5) and type V. Type III and non-typeable SCCmec were recovered to a lesser extent. All isolates were multi-resistant to at least two antimicrobials in addition to the expected cefoxitin and penicillin resistance, with an average of resistance to 9.5 different antimicrobials. Isolates selected for microarray analysis carried a broad range of antimicrobial resistance and virulence genes.
Conclusion
MRSA were mainly present in veal farms, compared to the lower prevalence in dairy or beef farms. Multi-resistance in these strains was high. Though mainly CC398 spa t011 was found, the genetic diversity was higher than what was found for pigs in Belgium. CC8 strains, a typically human lineage but also recently found also in association with bovines, has been retrieved here also.
doi:10.1186/1746-6148-10-153
PMCID: PMC4103977  PMID: 25011427
Nasal carriage; Bovine; Epidemiology; Molecular characterization; Antimicrobial resistance
10.  Methicillin-Resistant Staphylococcus aureus Colonization and Risk of Subsequent Infection in Critically Ill Children: Importance of Preventing Nosocomial Methicillin-Resistant Staphylococcus aureus Transmission 
Methicillin-resistant Staphylococcus aureus (MRSA) colonization is a risk factor for infection in critically ill children. Almost half of children who acquired MRSA colonization in our ICU developed an MRSA infection during their hospitalization or after discharge, highlighting the importance of preventing nosocomial MRSA transmission.
Background. Methicillin-resistant Staphylococcus aureus (MRSA) colonization is a predictor of subsequent infection in hospitalized adults. The risk of subsequent MRSA infections in hospitalized children colonized with MRSA is unknown.
Methods. Children admitted to an academic medical center’s pediatric intensive care unit between March 2007 and March 2010 were included in the study. Anterior naris swabs were cultured to identify children with MRSA colonization at admission. Laboratory databases were queried and National Healthcare Safety Network definitions applied to identify patients with MRSA infections during their hospitalization or after discharge.
Results. The MRSA admission prevalence among 3140 children was 4.9%. Overall, 56 children (1.8%) developed an MRSA infection, including 13 (8.5%) colonized on admission and 43 (1.4%) not colonized on admission (relative risk [RR], 5.9; 95% confidence interval [CI], 3.4–10.1). Of those, 10 children (0.3%) developed an MRSA infection during their hospitalization, including 3 of 153 children (1.9%) colonized on admission and 7 of 2987 children (0.2%) not colonized on admission (RR, 8.4; 95% CI, 2.7–25.8). African-Americans and those with public health insurance were more likely to get a subsequent infection (P < .01 and P = .03, respectively). We found that 15 children acquired MRSA colonization in the pediatric intensive care unit, and 7 (47%) developed a subsequent MRSA infection.
Conclusions. MRSA colonization is a risk factor for subsequent MRSA infection in children. Although MRSA colonized children may have lower risks of subsequent infection than adults, children who acquire MRSA in the hospital have similarly high rates of infection. Preventing transmission of MRSA in hospitalized children should remain a priority.
doi:10.1093/cid/cir547
PMCID: PMC3189167  PMID: 21878424
11.  Mupirocin-Resistant, Methicillin-Resistant Staphylococcus aureus Strains in Canadian Hospitals▿  
Antimicrobial Agents and Chemotherapy  2007;51(11):3880-3886.
Mupirocin resistance in Staphylococcus aureus is increasingly being reported in many parts of the world. This study describes the epidemiology and laboratory characterization of mupirocin-resistant methicillin-resistant S. aureus (MRSA) strains in Canadian hospitals. Broth microdilution susceptibility testing of 4,980 MRSA isolates obtained between 1995 and 2004 from 32 Canadian hospitals was done in accordance with CLSI guidelines. The clinical and epidemiologic characteristics of strains with high-level mupirocin resistance (HLMupr) were compared with those of mupirocin-susceptible (Mups) strains. MRSA strains were characterized by pulsed-field gel electrophoresis (PFGE) and typing of the staphylococcal chromosomal cassette mec. PCR was done to detect the presence of the mupA gene. For strains with mupA, plasmid DNA was extracted and subjected to Southern blot hybridization. A total of 198 (4.0%) HLMupr MRSA isolates were identified. The proportion of MRSA strains with HLMupr increased from 1.6% in the first 5 years of surveillance (1995 to 1999) to 7.0% from 2000 to 2004 (P < 0.001). Patients with HLMupr MRSA strains were more likely to have been aboriginal (odds ratio [OR], 3.7; 95% confidence interval [CI], 1.5 to 9.4; P = 0.006), to have had community-associated MRSA (OR, 2.2; 95% CI, 1.0 to 5.0; P = 0.05), and to have been colonized with MRSA (OR, 1.7; 95% CI, 1.0 to 3.0; P = 0.04). HLMupr MRSA strains were also more likely to be resistant to fusidic acid (21% versus 4% for mupirocin-susceptible strains; P < 0.001). All HLMupr MRSA strains had a plasmid-associated mupA gene, most often associated with a 9-kb HindIII fragment. PFGE typing and analysis of the plasmid profiles indicate that both plasmid transmission and the clonal spread of HLMupr MRSA have occurred in Canadian hospitals. These results indicate that the incidence of HLMupr is increasing among Canadian strains of MRSA and that HLMupr MRSA is recovered from patients with distinct clinical and epidemiologic characteristics compared to the characteristics of patents with Mups MRSA strains.
doi:10.1128/AAC.00846-07
PMCID: PMC2151460  PMID: 17724154
12.  Factors Associated with Nasal Colonization of Methicillin-Resistant Staphylococcus aureus among Healthy Children in Taiwan▿  
Journal of Clinical Microbiology  2010;49(1):131-137.
Methicillin-resistant Staphylococcus aureus (MRSA) has been identified as a major cause of community-associated (CA) S. aureus infections in the past decade. The main reservoir in the community for MRSA and the factors contributing to its worldwide spread remain poorly defined. Between July 2005 and June 2008, a total of 6,057 healthy children 2 to 60 months of age were screened for carriage of S. aureus and Streptococcus pneumoniae in Taiwan. The prevalence and epidemiological factors influencing MRSA carriage were determined. MRSA strains were tested for antimicrobial susceptibility and underwent molecular characterization. The overall prevalences of MRSA and S. aureus carriage were 7.8% and 23.2%, respectively. A majority (88%) of MRSA isolates belonged to a common Asian-Pacific CA-MRSA lineage, multilocus sequence type 59, and were resistant to multiple non-beta-lactam antibiotics. The carriage rate of MRSA was higher among subjects 2 to 6 months old (P < 0.0001), residing in northern Taiwan (P = 0.0003), and enrolled later in the study (P < 0.0001). MRSA colonization was associated with the number of children in the family (adjusted odds ratio [aOR], 1.114; 95% confidence interval [CI], 1.002 to 1.240; P = 0.0463) and day care attendance (aOR, 1.530; 95% CI, 1.201 to 1.949; P = 0.0006). Breast feeding (P < 0.0001) and colonization with S. pneumoniae (P = 0.0170) were protective against MRSA colonization. We concluded that epidemic CA-MRSA strains increasingly colonized Taiwanese children between 2005 and 2008. The carriage rate varied significantly across different demographical features. Crowding was an independent environmental risk factor that might accelerate CA-MRSA transmission in the community.
doi:10.1128/JCM.01774-10
PMCID: PMC3020448  PMID: 21084507
13.  Frequency of resistance to methicillin and other antimicrobial agents among Staphylococcus aureus strains isolated from pigs and their human handlers in Trinidad 
Infection Ecology & Epidemiology  2014;4:10.3402/iee.v4.22736.
Background
Methicillin-resistant Staphylococcus aureus (MRSA) has emerged recently worldwide in production animals, particularly pigs and veal calves, which act as reservoirs for MRSA strains for human infection. The study determined the prevalence of MRSA and other resistant strains of S. aureus isolated from the anterior nares of pigs and human handlers on pig farms in Trinidad.
Methods
Isolation of S. aureus was done by concurrently inoculating Baird-Parker agar (BPA) and Chromagar MRSA (CHROM) with swab samples and isolates were identified using standard methods. Suspect MRSA isolates from Chromagar and BPA were subjected to confirmatory test using Oxoid PBP2 latex agglutination test. The disc diffusion method was used to determine resistance to antimicrobial agents.
Results
The frequency of isolation of MRSA was 2.1% (15 of 723) for pigs but 0.0% (0 of 72) for humans. Generally, for isolates of S. aureus from humans there was a high frequency of resistance compared with those from pigs, which had moderate resistance to the following antimicrobials: penicillin G (54.5%, 51.5%), ampicillin (59.1%, 49.5%), and streptomycin (59.1%, 37.1%), respectively. There was moderate resistance to tetracycline (36.4%, 41.2%) and gentamycin (27.2%, 23.7%) for human and pig S. aureus isolates, respectively, and low resistance to sulfamethoxazole-trimethoprim (4.5%, 6.2%) and norfloxacin (9.1%, 12.4%), respectively. The frequency of resistance to oxacillin by the disc method was 36.4 and 34.0% from S. aureus isolates from humans and pigs, respectively. Out of a total of 78 isolates of S. aureus from both human and pig sources that were resistant to oxacillin by the disc diffusion method, only 15 (19.2%) were confirmed as MRSA by the PBP'2 latex test kit.
Conclusions
The detection of MRSA strains in pigs, albeit at a low frequency, coupled with a high frequency of resistance to commonly used antimicrobial agents in pig and humans could have zoonotic and therapeutic implications. Finally, the diagnostic limitation of using CHROMagar and testing for oxacillin resistance by the disc diffusion method alone to determine MRSA strains without performing confirmatory tests cannot be overemphasized because the possibility of overdiagnosis of MRSA infections cannot be ignored.
doi:10.3402/iee.v4.22736
PMCID: PMC3974178  PMID: 24765251
Staphylococcus aureus; MRSA; methicillin; pigs; resistance; Trinidad
14.  Transmission Dynamics of Methicillin-Resistant Staphylococcus aureus in Pigs 
From the mid-2000s on, numerous studies have shown that methicillin-resistant Staphylococcus aureus (MRSA), renowned as human pathogen, has a reservoir in pigs and other livestock. In Europe and North America, clonal complex (CC) 398 appears to be the predominant lineage involved. Especially worrisome is its capacity to contaminate humans in close contact with affected animals. Indeed, the typical multi-resistant phenotype of MRSA CC398 and its observed ability of easily acquiring genetic material suggests that MRSA CC398 strains with an increased virulence potential may emerge, for which few therapeutic options would remain. This questions the need to implement interventions to control the presence and spread of MRSA CC398 among pigs. MRSA CC398 shows a high but not fully understood transmission potential in the pig population and is able to persist within that population. Although direct contact is probably the main route for MRSA transmission between pigs, also environmental contamination, the presence of other livestock, the herd size, and farm management are factors that may be involved in the dissemination of MRSA CC398. The current review aims at summarizing the research that has so far been done on the transmission dynamics and risk factors for introduction and persistence of MRSA CC398 in farms.
doi:10.3389/fmicb.2013.00057
PMCID: PMC3602589  PMID: 23518663
MRSA; ST398; pigs; transmission risk factors; transmission routes; transmission pig models
15.  Alarming Proportions of Methicillin-Resistant Staphylococcus aureus (MRSA) in Wound Samples from Companion Animals, Germany 2010–2012 
PLoS ONE  2014;9(1):e85656.
Staphylococcus (S.) aureus is an important cause of wound infections in companion animals, and infections with methicillin-resistant S. aureus (MRSA) are of particular concern due to limited treatment options and their zoonotic potential. However, comparable epidemiological data on MRSA infections in dogs, cats and horses is scarce, also limiting the knowledge about possible links to MRSA isolates from human populations. To gain more knowledge about the occurrence and genotypic variation of MRSA among wound swabs of companion animal origin in Germany we performed a survey (2010–2012) including 5,229 samples from 1,170 veterinary practices. S. aureus was identified in 201 (5.8%) canine, 140 (12.2%) feline and 138 (22.8%) equine swabs from a total of 3,479 canine, 1,146 feline and 604 equine wounds, respectively. High MRSA rates were identified with 62.7%, 46.4% and 41.3% in S. aureus of canine, feline and equine origin, respectively. Further genotyping including spa typing and multilocus sequence typing (MLST) revealed a comparable distribution of spa types among canine and feline MRSA with CC22 (47.6%; 49.2%) and CC5 (30.2%; 29.2%) as predominant lineages followed by CC398 (13.5%; 7.7%) and CC8 (4.0%; 9.2%). In contrast, the majority of equine MRSA belonged to CC398 (87.7%). Our data highlight the importance of S. aureus and MRSA as a cause of wound infections, particularly in cats and horses in Germany. While “human-associated” MRSA lineages were most common in dogs and cats, a remarkable number of CC398-MRSA was detected in horses, indicating a replacement of CC8-MRSA as the predominant lineage within horses in Germany. These data enforce further longitudinal epidemiological approaches to examine the diversity and temporal relatedness of MRSA populations in humans and animals to assess probable sources of MRSA infections. This would enable a sound risk assessment and establishment of intervention strategies to limit the additional spread of MRSA.
doi:10.1371/journal.pone.0085656
PMCID: PMC3896405  PMID: 24465637
16.  Transmission of MRSA between Companion Animals and Infected Human Patients Presenting to Outpatient Medical Care Facilities 
PLoS ONE  2011;6(11):e26978.
Methicillin-resistant Staphylococcus aureus (MRSA) is a significant pathogen in both human and veterinary medicine. The importance of companion animals as reservoirs of human infections is currently unknown. The companion animals of 49 MRSA-infected outpatients (cases) were screened for MRSA carriage, and their bacterial isolates were compared with those of the infected patients using Pulsed-Field Gel Electrophoresis (PFGE). Rates of MRSA among the companion animals of MRSA-infected patients were compared to rates of MRSA among companion animals of pet guardians attending a “veterinary wellness clinic” (controls). MRSA was isolated from at least one companion animal in 4/49 (8.2%) households of MRSA-infected outpatients vs. none of the pets of the 50 uninfected human controls. Using PFGE, patient-pets MRSA isolates were identical for three pairs and discordant for one pair (suggested MRSA inter-specie transmission p-value = 0.1175). These results suggest that companion animals of MRSA-infected patients can be culture-positive for MRSA, representing a potential source of infection or re-infection for humans. Further studies are required to better understand the epidemiology of MRSA human-animal inter-specie transmission.
doi:10.1371/journal.pone.0026978
PMCID: PMC3213111  PMID: 22102871
17.  Methicillin-Resistant Staphylococcus aureus Associated with Animals and Its Relevance to Human Health 
Staphylococcus aureus is a typical human pathogen. Some animal S. aureus lineages have derived from human strains following profound genetic adaptation determining a change in host specificity. Due to the close relationship of animals with the environmental microbiome and resistome, animal staphylococcal strains also represent a source of resistance determinants. Methicillin-resistant S. aureus (MRSA) emerged 50 years ago as a nosocomial pathogen but in the last decade it has also become a frequent cause of infections in the community. The recent finding that MRSA frequently colonizes animals, especially livestock, has been a reason for concern, as it has revealed an expanded reservoir of MRSA. While MRSA strains recovered from companion animals are generally similar to human nosocomial MRSA, MRSA strains recovered from food animals appear to be specific animal-adapted clones. Since 2005, MRSA belonging to ST398 was recognized as a colonizer of pigs and human subjects professionally exposed to pig farming. The “pig” MRSA was also found to colonize other species of farmed animals, including horses, cattle, and poultry and was therefore designated livestock-associated (LA)-MRSA. LA-MRSA ST398 can cause infections in humans in contact with animals, and can infect hospitalized people, although at the moment this occurrence is relatively rare. Other animal-adapted MRSA clones have been detected in livestock, such as ST1 and ST9. Recently, ST130 MRSA isolated from bovine mastitis has been found to carry a novel mecA gene that eludes detection by conventional PCR tests. Similar ST130 strains have been isolated from human infections in UK, Denmark, and Germany at low frequency. It is plausible that the increased attention to animal MRSA will reveal other strains with peculiar characteristics that can pose a risk to human health.
doi:10.3389/fmicb.2012.00127
PMCID: PMC3321498  PMID: 22509176
Staphylococcus aureus; MRSA; ST398; animals; spa type
18.  In Vitro and In Vivo Evaluations of the Activities of Lauric Acid Monoester Formulations against Staphylococcus aureus 
Due to increasing mupirocin resistance, alternatives for Staphylococcus aureus nasal decolonization are needed. Lauric acid monoesters combined with lactic, mandelic, malic, or benzoic acid are being evaluated as possible alternatives. We determined the in vitro activity of 13 lauric acid monoester (LAM) formulations and mupirocin against 30 methicillin-susceptible S. aureus (MSSA) isolates and 30 methicillin-resistant S. aureus (MRSA) isolates. We then used a murine model of MRSA nasopharyngeal colonization to compare the in vivo activity of mupirocin with three LAM formulations. MSSA and MRSA MIC90 values were 0.25 μg/ml for mupirocin and ≤4 μl/ml for all LAM formulations tested. Hsd:ICR mice were challenged with 108 CFU/naris MRSA. Five days later, S. aureus colonization was documented by culture. Treatment with bland, mupirocin, or one of three LAM ointments was then administered unblinded thrice daily for 2 days. Three days after treatment, both anterior nares were cultured for S. aureus. Administration of 128774-49E or 128774-53A was associated with greater eradication of MRSA carriage (24/34 [71%] or 33/40 [83%]) of animals, respectively) than bland ointment (12/38 [32%]) (P < 0.005). 128774-53A administration resulted in greater MRSA carriage eradication than mupirocin (19/38 [50%]) (P < 0.005) in this model. LAM formulations warrant evaluation for S. aureus nasal decolonization in humans.
doi:10.1128/AAC.49.8.3187-3191.2005
PMCID: PMC1196268  PMID: 16048923
19.  Automated Detection of Infectious Disease Outbreaks in Hospitals: A Retrospective Cohort Study 
PLoS Medicine  2010;7(2):e1000238.
Susan Huang and colleagues describe an automated statistical software, WHONET-SaTScan, its application in a hospital, and the potential it has to identify hospital infection clusters that had escaped routine detection.
Background
Detection of outbreaks of hospital-acquired infections is often based on simple rules, such as the occurrence of three new cases of a single pathogen in two weeks on the same ward. These rules typically focus on only a few pathogens, and they do not account for the pathogens' underlying prevalence, the normal random variation in rates, and clusters that may occur beyond a single ward, such as those associated with specialty services. Ideally, outbreak detection programs should evaluate many pathogens, using a wide array of data sources.
Methods and Findings
We applied a space-time permutation scan statistic to microbiology data from patients admitted to a 750-bed academic medical center in 2002–2006, using WHONET-SaTScan laboratory information software from the World Health Organization (WHO) Collaborating Centre for Surveillance of Antimicrobial Resistance. We evaluated patients' first isolates for each potential pathogenic species. In order to evaluate hospital-associated infections, only pathogens first isolated >2 d after admission were included. Clusters were sought daily across the entire hospital, as well as in hospital wards, specialty services, and using similar antimicrobial susceptibility profiles. We assessed clusters that had a likelihood of occurring by chance less than once per year. For methicillin-resistant Staphylococcus aureus (MRSA) or vancomycin-resistant enterococci (VRE), WHONET-SaTScan–generated clusters were compared to those previously identified by the Infection Control program, which were based on a rule-based criterion of three occurrences in two weeks in the same ward. Two hospital epidemiologists independently classified each cluster's importance. From 2002 to 2006, WHONET-SaTScan found 59 clusters involving 2–27 patients (median 4). Clusters were identified by antimicrobial resistance profile (41%), wards (29%), service (13%), and hospital-wide assessments (17%). WHONET-SaTScan rapidly detected the two previously known gram-negative pathogen clusters. Compared to rule-based thresholds, WHONET-SaTScan considered only one of 73 previously designated MRSA clusters and 0 of 87 VRE clusters as episodes statistically unlikely to have occurred by chance. WHONET-SaTScan identified six MRSA and four VRE clusters that were previously unknown. Epidemiologists considered more than 95% of the 59 detected clusters to merit consideration, with 27% warranting active investigation or intervention.
Conclusions
Automated statistical software identified hospital clusters that had escaped routine detection. It also classified many previously identified clusters as events likely to occur because of normal random fluctuations. This automated method has the potential to provide valuable real-time guidance both by identifying otherwise unrecognized outbreaks and by preventing the unnecessary implementation of resource-intensive infection control measures that interfere with regular patient care.
Please see later in the article for the Editors' Summary
Editors' Summary
Background
Admission to a hospital is often a life-saving necessity—individuals injured in a road accident, for example, may need immediate medical and surgical attention if they are to survive. Unfortunately, many patients acquire infections, some of which are life-threatening, during their stay in a hospital. The World Health Organization has estimated that, globally, 8.7% of hospital patients develop hospital-acquired infections (infections that are identified more than two days after admission to hospital). In the US alone, 2 million people develop a hospital-acquired infection every year, often an infection of a surgical wound, or a urinary tract or lung infection. Infections are common among hospital patients because increasing age or underlying illnesses can reduce immunity to infection and because many medical and surgical procedures bypass the body's natural protective barriers. In addition, poor infection control practices can facilitate the transmission of bacteria—including meticillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE)—and other infectious agents (pathogens) between patients.
Why Was This Study Done?
Sometimes, the number of cases of hospital-acquired infections increases unexpectedly or a new infection emerges. Such clusters account for relatively few health care–associated infections, but, because they may arise from the transmission of a pathogen within a hospital, they need to be rapidly identified and measures implemented (for example, isolation of affected patients) to stop transmission if an outbreak is confirmed. Currently, the detection of clusters of hospital-acquired infections is based on simple rules, such as the occurrence of three new cases of a single pathogen in two weeks on the same ward. This rule-based approach relies on the human eye to detect infection clusters within microbiology data (information collected on the pathogens isolated from patients), it focuses on a few pathogens, and it does not consider the random variation in infection rates or the possibility that clusters might be associated with shared facilities rather than with individual wards. In this study, the researchers test whether an automated statistical system can detect outbreaks of hospital-acquired infections quickly and accurately.
What Did the Researchers Do and Find?
The researchers combined two software packages used to track diseases in populations to create the WHONET-SaTScan cluster detection tool. They then compared the clusters of hospital-acquired infection identified by the new tool in microbiology data from a 750-bed US academic medical center with those generated by the hospital's infection control program, which was largely based on the simple rule described above. WHONET-SaTScan found 59 clusters of infection that occurred between 2002 and 2006, about three-quarters of which were identified by characteristics other than a ward-based location. Nearly half the cluster alerts were generated on the basis of shared antibiotic susceptibility patterns. Although WHONET-SaTScan identified all the clusters previously identified by the hospital's infection control program, it classified most of these clusters as likely to be the result of normal random variations in infection rates rather than the result of “true” outbreaks. By contrast, the hospital's infection control department only identified three of the 59 statistically significant clusters identified by WHONET-SaTScan. Furthermore, the new tool identified six previously unknown MRSA outbreaks and four previously unknown VRE outbreaks. Finally, two hospital epidemiologists (scientists who study diseases in populations) classified 95% of the clusters detected by WHONET-SaTScan as worthy of consideration by the hospital infection control team and a quarter of the clusters as warranting active investigation or intervention.
What Do These Findings Mean?
These findings suggest that automated statistical software should be able to detect clusters of hospital-acquired infections that would escape detection using routine rule-based systems. Importantly, they also suggest that an automated system would be able to discount a large number of supposed outbreaks identified by rule-based systems. These findings need to be confirmed in other settings and in prospective studies in which the outcomes of clusters detected with WHONET-SaTScan are carefully analyzed. For now, however, these findings suggest that automated statistical tools could provide hospital infection control experts with valuable real-time guidance by identifying outbreaks that would be missed by routine detection methods and by preventing the implementation of intensive and costly infection control measures in situations where they are unnecessary.
Additional Information
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000238.
The World Health Organization's Prevention of Hospital-Acquired Infections, A Practical Guide contains detailed information on all aspects of hospital-acquired infections
MedlinePlus provides links to information on infection control in hospitals (in English and Spanish)
The US Centers for Disease Control and Prevention also provides information on infectious diseases in health care settings (in English and Spanish)
The WHONET/Baclink software and the SatScan software, the two components of WHONET-SaTScan are both available on the internet (the WHONET-SaTScan cluster detection tool is freely available as part of the version of WHONET/BacLink released June 2009)
doi:10.1371/journal.pmed.1000238
PMCID: PMC2826381  PMID: 20186274
20.  The use of the temporal scan statistic to detect methicillin-resistant Staphylococcus aureus clusters in a community hospital 
BMC Infectious Diseases  2014;14:375.
Background
In healthcare facilities, conventional surveillance techniques using rule-based guidelines may result in under- or over-reporting of methicillin-resistant Staphylococcus aureus (MRSA) outbreaks, as these guidelines are generally unvalidated. The objectives of this study were to investigate the utility of the temporal scan statistic for detecting MRSA clusters, validate clusters using molecular techniques and hospital records, and determine significant differences in the rate of MRSA cases using regression models.
Methods
Patients admitted to a community hospital between August 2006 and February 2011, and identified with MRSA > 48 hours following hospital admission, were included in this study. Between March 2010 and February 2011, MRSA specimens were obtained for spa typing. MRSA clusters were investigated using a retrospective temporal scan statistic. Tests were conducted on a monthly scale and significant clusters were compared to MRSA outbreaks identified by hospital personnel. Associations between the rate of MRSA cases and the variables year, month, and season were investigated using a negative binomial regression model.
Results
During the study period, 735 MRSA cases were identified and 167 MRSA isolates were spa typed. Nine different spa types were identified with spa type 2/t002 (88.6%) the most prevalent. The temporal scan statistic identified significant MRSA clusters at the hospital (n = 2), service (n = 16), and ward (n = 10) levels (P ≤ 0.05). Seven clusters were concordant with nine MRSA outbreaks identified by hospital staff. For the remaining clusters, seven events may have been equivalent to true outbreaks and six clusters demonstrated possible transmission events. The regression analysis indicated years 2009–2011, compared to 2006, and months March and April, compared to January, were associated with an increase in the rate of MRSA cases (P ≤ 0.05).
Conclusions
The application of the temporal scan statistic identified several MRSA clusters that were not detected by hospital personnel. The identification of specific years and months with increased MRSA rates may be attributable to several hospital level factors including the presence of other pathogens. Within hospitals, the incorporation of the temporal scan statistic to standard surveillance techniques is a valuable tool for healthcare workers to evaluate surveillance strategies and aid in the identification of MRSA clusters.
doi:10.1186/1471-2334-14-375
PMCID: PMC4097048  PMID: 25005247
Methicillin-resistant Staphylococcus aureus; Clusters; Temporal scan statistic; Community hospital; Epidemiology; Spa typing
21.  Molecular epidemiology of clinical and carrier strains of methicillin resistant Staphylococcus aureus (MRSA) in the hospital settings of north India 
Background
The study was conducted between 2000 and 2003 on 750 human subjects, yielding 850 strains of staphylococci from clinical specimens (575), nasal cultures of hospitalized patients (100) and eye & nasal sources of hospital workers (50 & 125 respectively) in order to determine their epidemiology, acquisition and dissemination of resistance genes.
Methods
Organisms from clinical samples were isolated, cultured and identified as per the standard routine procedures. Susceptibility was measured by the agar diffusion method, as recommended by the Nat ional Committee for Clinical Laboratory Standards (NCCLS). The modified method of Birnboin and Takahashi was used for isolation of plasmids from staphylococci. Pulsed-field gel electrophoresis (PFGE) typing of clinical and carrier Methicillin resistant Staphylococcus aureus (MRSA) strains isolated during our study was performed as described previously.
Results
It was shown that 35.1% of Staphylococcus aureus and 22.5% of coagulase-negative staphylococcal isolates were resistant to methicillin. Highest percentage of MRSA (35.5%) was found in pus specimens (n = 151). The multiple drug resistance of all MRSA (n = 180) and Methicillin resistant Coagulase-negative Staphylococcus aureus (MRCNS) (n = 76) isolates was detected. In case of both methicillin-resistant as well as methicillin-sensitive Saphylococcal isolates zero resistance was found to vancomycin where as highest resistance was found to penicillin G followed by ampicillin. It was shown that the major reservoir of methicillin resistant staphylococci in hospitals are colonized/infected inpatients and colonized hospital workers, with carriers at risk for developing endogenous infection or transmitting infection to health care workers and patients. The results were confirmed by molecular typing using PFGE by SmaI-digestion.
It was shown that the resistant markers G and T got transferred from clinical S. aureus (JS-105) to carrier S. aureus (JN-49) and the ciprofloxacin (Cf) and erythromycin (E) resistance seemed to be chromosomal mediated. In one of the experiments, plasmid pJMR1O from Staphylococcus aureus coding for ampicillin (A), gentamicin (G) and amikacin (Ak) resistance was transformed into Escherichia coli. The minimal inhibitory concentrations (MICs) for A and G were lower in E. coli than in S. aureus. However, the MIC for Ak was higher in E. coli transformants than in S. aureus.
Conclusion
There is a progressive increase in MRSA prevalence and multi-drug resistance in staphylococci. Vancomycin is still the drug of choice for MRSA infections. The major reservoir of methicillin resistant staphylococci in hospitals is colonized/infected inpatients and colonized hospital workers. Resistance transfer from staphylococci to E. coli as well as from clinical to carrier staphylococci due to antibiotic stress seemed to be an alarming threat to antimicrobial chemotherapy.
doi:10.1186/1476-0711-5-22
PMCID: PMC1592298  PMID: 16972997
22.  Prevalence and antimicrobial susceptibility pattern of methicillin resistant Staphylococcus aureus isolates from Trinidad & Tobago 
Background
Methicillin-resistant Staphylococcus aureus (MRSA) has become increasingly prevalent worldwide since it was first reported in a British hospital. The prevalence however, varies markedly in hospitals in the same country, and from one country to another. We therefore sought to document comprehensively the prevalence and antimicrobial susceptibility pattern of MRSA isolates in Trinidad and Tobago.
Methods
All Staphylococcus aureus isolates encountered in routine clinical specimens received at major hospitals in the country between 2000 and 2001 were identified morphologically and biochemically by standard laboratory procedures including latex agglutination test (Staphaurex Plus; Murex Diagnostics Ltd; Dartford, England); tube coagulase test with rabbit plasma (Becton, Dickinson & Co; Sparks, MD, USA), and DNase test using DNase agar (Oxoid Ltd; Basingstoke, Hampshire, England). MRSA screening was performed using Mueller-Hinton agar containing 6 μg oxacillin and 4% NaCl, latex agglutination test (Denka Seiken Co. Ltd, Tokyo, Japan) and E-test system (AB Biodisk, Solna, Sweden). Susceptibility to antimicrobial agents was determined by the modified Kirby Bauer disc diffusion method while methicillin MICs were determined with E-test system.
Results
Of 1,912 S. aureus isolates received, 12.8% were methicillin (oxacillin) resistant. Majority of the isolates were recovered from wound swabs (86.9%) and the least in urine (0.4%) specimens. Highest number of isolates was encountered in the surgical (62.3%) and the least from obstetrics and gynaecology (1.6%) facilities respectively. Large proportions of methicillin sensitive isolates are >85% sensitive to commonly used and available antimicrobials in the country. All MRSA isolates were resistant to ceftriaxone, erythromycin, gentamicin and penicillin but were 100% sensitive to vancomycin, rifampin and chloramphenicol.
Conclusion
There is a progressive increase in MRSA prevalence in the country but the present rate is still low in comparison to values in some other countries. Vancomycin is still the drug of choice for treating multidrug resistant MRSA infections. Further use of molecular studies to monitor the epidemiology of MRSA in these hospitals in the country is highly recommended too.
doi:10.1186/1476-0711-5-16
PMCID: PMC1543645  PMID: 16817961
23.  A Shared Population of Epidemic Methicillin-Resistant Staphylococcus aureus 15 Circulates in Humans and Companion Animals 
mBio  2014;5(3):e00985-13.
ABSTRACT
Methicillin-resistant Staphylococcus aureus (MRSA) is a global human health problem causing infections in both hospitals and the community. Companion animals, such as cats, dogs, and horses, are also frequently colonized by MRSA and can become infected. We sequenced the genomes of 46 multilocus sequence type (ST) 22 MRSA isolates from cats and dogs in the United Kingdom and compared these to an extensive population framework of human isolates from the same lineage. Phylogenomic analyses showed that all companion animal isolates were interspersed throughout the epidemic MRSA-15 (EMRSA-15) pandemic clade and clustered with human isolates from the United Kingdom, with human isolates basal to those from companion animals, suggesting a human source for isolates infecting companion animals. A number of isolates from the same veterinary hospital clustered together, suggesting that as in human hospitals, EMRSA-15 isolates are readily transmitted in the veterinary hospital setting. Genome-wide association analysis did not identify any host-specific single nucleotide polymorphisms (SNPs) or virulence factors. However, isolates from companion animals were significantly less likely to harbor a plasmid encoding erythromycin resistance. When this plasmid was present in animal-associated isolates, it was more likely to contain mutations mediating resistance to clindamycin. This finding is consistent with the low levels of erythromycin and high levels of clindamycin used in veterinary medicine in the United Kingdom. This study furthers the “one health” view of infectious diseases that the pathogen pool of human and animal populations are intrinsically linked and provides evidence that antibiotic usage in animal medicine is shaping the population of a major human pathogen.
IMPORTANCE
Methicillin-resistant Staphylococcus aureus (MRSA) is major problem in human medicine. Companion animals, such as cats, dogs, and horses, can also become colonized and infected by MRSA. Here, we demonstrate that a shared population of an important and globally disseminated lineage of MRSA can infect both humans and companion animals without undergoing host adaptation. This suggests that companion animals might act as a reservoir for human infections. We also show that the isolates from companion animals have differences in the presence of certain antibiotic resistance genes. This study furthers the “one health” view of infectious diseases by demonstrating that the pool of MRSA isolates in the human and animal populations are shared and highlights how different antibiotic usage patterns between human and veterinary medicine can shape the population of bacterial pathogens.
doi:10.1128/mBio.00985-13
PMCID: PMC4030480  PMID: 24825010
24.  Extensive Genetic Diversity Identified among Sporadic Methicillin-Resistant Staphylococcus aureus Isolates Recovered in Irish Hospitals between 2000 and 2012 
Clonal replacement of predominant nosocomial methicillin-resistant Staphylococcus aureus (MRSA) strains has occurred several times in Ireland during the last 4 decades. However, little is known about sporadically occurring MRSA in Irish hospitals or in other countries. Eighty-eight representative pvl-negative sporadic MRSA isolates recovered in Irish hospitals between 2000 and 2012 were investigated. These yielded unusual pulsed-field gel electrophoresis and antibiogram-resistogram typing patterns distinct from those of the predominant nosocomial MRSA clone, ST22-MRSA-IV, during the study period. Isolates were characterized by spa typing and DNA microarray profiling for multilocus sequence type (MLST) clonal complex (CC) and/or sequence type (ST) and SCCmec type assignment, as well as for detection of virulence and antimicrobial resistance genes. Conventional PCR-based SCCmec subtyping was undertaken when necessary. Extensive diversity was detected, including 38 spa types, 13 MLST-CCs (including 18 STs among 62 isolates assigned to STs), and 25 SCCmec types (including 2 possible novel SCCmec elements and 7 possible novel SCCmec subtypes). Fifty-four MLST-spa-SCCmec type combinations were identified. Overall, 68.5% of isolates were assigned to nosocomial lineages, with ST8-t190-MRSA-IID/IIE ± SCCM1 predominating (17.4%), followed by CC779/ST779-t878-MRSA-ψSCCmec-SCC-SCCCRISPR (7.6%) and CC22/ST22-t032-MRSA-IVh (5.4%). Community-associated clones, including CC1-t127/t386/t2279-MRSA-IV, CC59-t216-MRSA-V, CC8-t008-MRSA-IVa, and CC5-t002/t242-MRSA-IV/V, and putative animal-associated clones, including CC130-t12399-MRSA-XI, ST8-t064-MRSA-IVa, ST398-t011-MRSA-IVa, and CC6-t701-MRSA-V, were also identified. In total, 53.3% and 47.8% of isolates harbored genes for resistance to two or more classes of antimicrobial agents and two or more mobile genetic element-encoded virulence-associated factors, respectively. Effective ongoing surveillance of sporadic nosocomial MRSA is warranted for early detection of emerging clones and reservoirs of virulence, resistance, and SCCmec genes.
doi:10.1128/AAC.02653-13
PMCID: PMC4023797  PMID: 24395241
25.  Potential for pet animals to harbor methicillin-resistant Staphylococcus aureus (MRSA) when residing with human MRSA patients 
Zoonoses and Public Health  2012;59(4):286-293.
Summary
Colonization by methicillin-resistant Staphylococcus aureus (MRSA) may be persistent in people, and is horizontally transmissible. The scientific literature suggests that domestic pets may also participate in cross-transmission of MRSA within households. The objectives of this study were to evaluate the prevalence of and risk factors for MRSA carriage by pets residing in households with an MRSA-infected person. From 66 households in which an MRSA infected patient resided, we screened 47 dogs and 52 cats using a swab protocol. Isolates from pets and humans were genotyped using two techniques, and compared for concordance. Human participants completed a 22-question survey of demographic and epidemiologic data relevant to staphylococcal transmission. Eleven of 99 pets (11.5%) representing 9 (13.6%) of households were MRSA-positive, but in only 6 of these households were the human and animal-source strains genetically concordant. Human infection by strain USA 100 was significantly associated with pet carriage [OR = 11.4 (95% C.I. 1.7, 76.9); p=0.013]. Yet, for each day of delay in sampling the pet after the person’s MRSA diagnosis, the odds of isolating any type of MRSA from the pet decreased by 13.9% [(95% C.I. 2.6%, 23.8%); p=0.017)]. It may be concluded that pets can harbor pandemic strains of MRSA while residing in a household with an infected person. However, the source of MRSA to the pet cannot always be attributed to the human patient. Moreover, the rapid attrition of the odds of obtaining a positive culture from pets over time suggests that MRSA carriage may be fleeting.
doi:10.1111/j.1863-2378.2011.01448.x
PMCID: PMC3326176  PMID: 22233337
Pets; Staphylococcus; MRSA; zoonosis; Bacterial Typing Techniques

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