Search tips
Search criteria

Results 1-25 (67)

Clipboard (0)

Select a Filter Below

Year of Publication
more »
1.  Virulence Plasmids of Nonsporulating Gram-Positive Pathogens 
Microbiology spectrum  2014;2(5):10.1128/microbiolspec.PLAS-0002-2013.
Gram-positive bacteria are leading causes of many types of human infection, including pneumonia, skin and nasopharyngeal infections, as well as urinary tract and surgical wound infections among hospitalized patients. These infections have become particularly problematic because many of the species causing them have become highly resistant to antibiotics. The role of mobile genetic elements, such as plasmids, in the dissemination of antibiotic resistance among Gram-positive bacteria has been well studied; less well understood is the role of mobile elements in the evolution and spread of virulence traits among these pathogens. While these organisms are leading agents of infection, they are also prominent members of the human commensal ecology. It appears that these bacteria are able to take advantage of the intimate association between host and commensal, via virulence traits that exacerbate infection and cause disease. However, evolution into an obligate pathogen has not occurred, presumably because it would lead to rejection of pathogenic organisms from the host ecology. Instead, in organisms that exist as both commensal and pathogen, selection has favored the development of mechanisms for variability. As a result, many virulence traits are localized on mobile genetic elements, such as virulence plasmids and pathogenicity islands. Virulence traits may occur within a minority of isolates of a given species, but these minority populations have nonetheless emerged as a leading problem in infectious disease. This chapter reviews virulence plasmids in nonsporulating Gram-positive bacteria, and examines their contribution to disease pathogenesis.
PMCID: PMC4275793  PMID: 25544937
2.  In Vitro and In Vivo Models of Staphylococcus aureus Endophthalmitis Implicate Specific Nutrients in Ocular Infection 
PLoS ONE  2014;9(10):e110872.
To define global transcriptional responses of Staphylococcus aureus and its codY mutant (CodY is a transcription regulator of virulence and metabolic genes in response to branched-chain amino acids) when growing in bovine aqueous (AH) and vitreous humor (VH) in vitro, and to investigate the impact of codY deletion on S. aureus virulence in a novel murine anterior chamber (AC) infection model.
For the in vitro model, differential transcriptomic gene expression of S. aureus and its codY mutant grown in chemically defined medium (CDM), AH, and VH was analyzed. Furthermore, the strains were inoculated into the AC of mice. Changes in bacterial growth, electroretinography and inflammation scores were monitored.
Bovine AH and VH provide sufficient nutrition for S. aureus growth in vitro. Transcriptome analysis identified 72 unique open reading frames differentially regulated ≥10-fold between CDM, AH, and VH. In the AC model, we found comparable growth of the codY mutant and wild type strains in vivo. Average inflammation scores and retinal function were significantly worse for codY mutant-infected eyes at 24 h post-infection.
Our in vitro bovine AH and VH models identified likely nutrient sources for S. aureus in the ocular milieu. The in vivo model suggests that control of branched-chain amino acid availability has therapeutic potential in limiting S. aureus endophthalmitis severity.
PMCID: PMC4207797  PMID: 25340474
3.  An ABC Transporter Is Required for Secretion of Peptide Sex Pheromones in Enterococcus faecalis 
mBio  2014;5(5):e01726-14.
Enterococci are leading causes of hospital-acquired infection in the United States and continue to develop resistances to new antibiotics. Many Enterococcus faecalis isolates harbor pheromone-responsive plasmids that mediate horizontal transfer of even large blocks of chromosomal genes, resulting in hospital-adapted strains over a quarter of whose genomes consist of mobile elements. Pheromones to which the donor cells respond derive from lipoprotein signal peptides. Using a novel bacterial killing assay dependent on the presence of sex pheromones, we screened a transposon mutant library for functions that relate to the production and/or activity of the effector pheromone. Here we describe a previously uncharacterized, but well-conserved, ABC transporter that contributes to pheromone production. Using three distinct pheromone-dependent mating systems, we show that mutants defective in expressing this transporter display a 5- to 6-order-of-magnitude reduction in conjugation efficiency. In addition, we demonstrate that the ABC transporter mutant displays an altered biofilm architecture, with a significant reduction in biofilm biomass compared to that of its isogenic parent, suggesting that pheromone activity also influences biofilm development. The conservation of this peptide transporter across the Firmicutes suggests that it may also play an important role in cell-cell communication in other species within this important phylum.
Enterococcus faecalis ranks as one of the leading causes of hospital-associated infections. Strains possessing resistance to multiple antibiotics are becoming all too common in clinical settings. Pheromone-responsive plasmids play an important role in harboring and disseminating these antibiotic resistance genes. Here we have identified a novel ABC transporter that is responsible for the secretion of peptide pheromones, which enables communication between cells to mediate plasmid transfer. We have also shown that this transporter is important for biofilm formation, providing a strong rationale for its use as a viable therapeutic target which could be targeted to curb infection, as well as the spread of existing drug resistance.
PMCID: PMC4173765  PMID: 25249282
4.  Genes Contributing to Staphylococcus aureus Fitness in Abscess- and Infection-Related Ecologies 
mBio  2014;5(5):e01729-14.
Staphylococcus aureus is a leading cause of both community- and hospital-acquired infections that are increasingly antibiotic resistant. The emergence of S. aureus resistance to even last-line antibiotics heightens the need for the development of new drugs with novel targets. We generated a highly saturated transposon insertion mutant library in the genome of S. aureus and used Tn-seq analysis to probe the entire genome, with unprecedented resolution and sensitivity, for genes of importance in infection. We further identified genes contributing to fitness in various infected compartments (blood and ocular fluids) and compared them to genes required for growth in rich medium. This resulted in the identification of 426 genes that were important for S. aureus fitness during growth in infection models, including 71 genes that could be considered essential for survival specifically during infection. These findings highlight novel as well as previously known genes encoding virulence traits and metabolic pathways important for S. aureus proliferation at sites of infection, which may represent new therapeutic targets.
Staphylococcus aureus continues to be a leading cause of antibiotic-resistant community and nosocomial infection. With the bacterium’s acquisition of resistance to methicillin and, more recently, vancomycin, the need for the development of new drugs with novel targets is urgent. Applying a highly saturated Tn-seq mutant library to analyze fitness and growth requirements in a murine abscess and in various infection-relevant fluids, we identified S. aureus traits that enable it to survive and proliferate during infection. This identifies potential new targeting opportunities for the development of novel therapeutics.
PMCID: PMC4173792  PMID: 25182329
5.  Cost-Effectiveness of Alternating MRI and Digital Mammography Screening in BRCA1 and BRCA2 Gene Mutation Carriers 
Cancer  2012;119(6):1266-1276.
Current clinical guidelines recommend earlier, more intensive breast cancer screening with both MRI and mammography for women with BRCA mutations. Unspecified details of screening schedules are a challenge for implementing guidelines.
A Markov Monte Carlo computer model simulated screening in asymptomatic female BRCA1 and BRCA2 mutation carriers. Three dual-modality strategies were compared with digital mammography (DM) alone: 1) DM and MRI alternating at 6-month intervals beginning at age 25 [Alt25], 2) annual MRI beginning at age 25 with alternating DM added at age 30 [MRI25/Alt30], and 3) DM and MRI alternating at 6-month intervals beginning at age 30 [Alt30]. Primary outcomes were quality-adjusted life years (QALYs), lifetime costs (in 2010 USD), and incremental cost-effectiveness ($/QALY gained). Additional outcomes included potential harms of screening, and lifetime costs stratified into component categories (screening and diagnosis, treatment, mortality, and patient time costs).
All three dual-modality screening strategies increased QALYs and costs. Alt30 screening had the lowest incremental costs per additional QALY gained: (BRCA1: $74,200/QALY; BRCA2: $215,700/QALY). False-positive test results increased substantially with dual-modality screening, occurring more frequently in BRCA2 carriers. Downstream savings in both breast cancer treatment and mortality costs were outweighed by increases in up-front screening and diagnosis costs. Results were most influenced by estimates of breast cancer risk and MRI cost.
Alternating MRI and DM screening at 6-month intervals beginning at age 30 is a clinically effective approach to applying current guidelines, and is more cost-effective in BRCA1 compared with BRCA2 gene mutation carriers.
PMCID: PMC3586945  PMID: 23184400
BRCA1 gene; BRCA2 gene; breast neoplasms; cancer screening; cost effectiveness
6.  Genomic Transition of Enterococci from Gut Commensals to Leading Causes of Multidrug-resistant Hospital Infection in the Antibiotic Era 
The enterococci evolved over eons as highly adapted members of gastrointestinal consortia of a wide variety of hosts, but for reasons that are not entirely clear, emerged in the 1970s as leading causes of multidrug resistant hospital infection. Hospital-adapted pathogenic isolates are characterized by the presence of multiple mobile elements conferring antibiotic resistance, as well as pathogenicity islands, capsule loci and other variable traits. Enterococci may have been primed to emerge among the vanguard of antibiotic resistant strains because of their occurrence in the GI tracts of insects and simple organisms living and feeding on organic matter that is colonized by antibiotic resistant, antibiotic producing micro-organisms. In response to the opportunity to inhabit a new niche – the antibiotic treated hospital patient – the enterococcal genome is evolving in a pattern characteristic of other bacteria that have emerged as pathogens because of opportunities stemming from anthropogenic change.
PMCID: PMC3649759  PMID: 23395351
7.  High-Quality Draft Genome Sequence of Vagococcus lutrae Strain LBD1, Isolated from the Largemouth Bass Micropterus salmoides 
Genome Announcements  2013;1(6):e01087-13.
Vagococci are usually isolated from marine hosts and occasionally from endodontic infections. Using 16S rRNA gene comparison, the closest relatives are members of the genera Enterococcus and Carnobacterium. A draft sequence of Vagococcus lutrae was generated to clarify the relationship of Vagococcus to these and other related low-G+C Gram-positive bacteria.
PMCID: PMC3873611  PMID: 24371201
8.  Draft Genome Sequence of Methicillin-Resistant Staphylococcus aureus Strain SA16, Representative of an Endemic Clone from a Brazilian Hospital 
Genome Announcements  2013;1(5):e00754-13.
Here we report the draft genome sequence of a bloodstream isolate of methicillin-resistant Staphylococcus aureus strain SA16. Strain SA16 is a sequence type 5 (ST5)-staphylococcal cassette chromosome mec type II (SCCmec II) clone and was the most prevalent isolate at a Brazilian hospital during the second half of 2009.
PMCID: PMC3778207  PMID: 24051324
9.  Staphylococcus aureus Activates the NLRP3 Inflammasome in Human and Rat Conjunctival Goblet Cells 
PLoS ONE  2013;8(9):e74010.
The conjunctiva is a moist mucosal membrane that is constantly exposed to an array of potential pathogens and triggers of inflammation. The NACHT, leucine rich repeat (LRR), and pyrin domain-containing protein 3 (NLRP3) is a Nod-like receptor that can sense pathogens or other triggers, and is highly expressed in wet mucosal membranes. NLRP3 is a member of the multi-protein complex termed the NLRP3 inflammasome that activates the caspase 1 pathway, inducing the secretion of biologically active IL-1β, a major initiator and promoter of inflammation. The purpose of this study was to: (1) determine whether NLRP3 is expressed in the conjunctiva and (2) determine whether goblet cells specifically contribute to innate mediated inflammation via secretion of IL-1β. We report that the receptors known to be involved in the priming and activation of the NLRP3 inflammasome, the purinergic receptors P2X4 and P2X7 and the bacterial Toll-like receptor 2 are present and functional in conjunctival goblet cells. Toxin-containing Staphylococcus aureus (S. aureus), which activates the NLRP3 inflammasome, increased the expression of the inflammasome proteins NLRP3, ASC and pro- and mature caspase 1 in conjunctival goblet cells. The biologically active form of IL-1β was detected in goblet cell culture supernatants in response to S. aureus, which was reduced when the cells were treated with the caspase 1 inhibitor Z-YVAD. We conclude that the NLRP3 inflammasome components are present in conjunctival goblet cells. The NRLP3 inflammasome appears to be activated in conjunctival goblet cells by toxin-containing S. aureus via the caspase 1 pathway to secrete mature IL1-β. Thus goblet cells contribute to the innate immune response in the conjunctiva by activation of the NLRP3 inflammasome.
PMCID: PMC3769353  PMID: 24040145
10.  Wall teichoic acid protects Staphylococcus aureus from inhibition by Congo red and other dyes 
Polyanionic polymers, including lipoteichoic acid and wall teichoic acid, are important determinants of the charged character of the staphylococcal cell wall. This study was designed to investigate the extent to which teichoic acid contributes to protection from anionic azo dyes and to identify barriers to drug penetration for development of new antibiotics for multidrug-resistant Staphylococcus aureus infection.
We studied antimicrobial activity of azo dyes against S. aureus strains with or without inhibition of teichoic acid in vitro and in vivo.
We observed that inhibition of wall teichoic acid expression resulted in an ∼1000-fold increase in susceptibility to azo dyes such as Congo red, reducing its MIC from >1024 to <4 mg/L. Sensitization occurred when the first step in the wall teichoic acid pathway, catalysed by TarO, was inhibited either by mutation or by chemical inhibition. In contrast, genetic blockade of lipoteichoic acid biosynthesis did not confer Congo red susceptibility. Based on this finding, combination therapy was tested using the highly synergistic combination of Congo red plus tunicamycin at sub-MIC concentrations (to inhibit wall teichoic acid biosynthesis). The combination rescued Caenorhabditis elegans from a lethal challenge of S. aureus.
Our studies show that wall teichoic acid confers protection to S. aureus from anionic azo dyes and related compounds, and its inhibition raises the prospect of development of new combination therapies based on this inhibition.
PMCID: PMC3584970  PMID: 22615298
bacteria; antibiotics; S. aureus
11.  Emergence of Epidemic Multidrug-Resistant Enterococcus faecium from Animal and Commensal Strains 
mBio  2013;4(4):e00534-13.
Enterococcus faecium, natively a gut commensal organism, emerged as a leading cause of multidrug-resistant hospital-acquired infection in the 1980s. As the living record of its adaptation to changes in habitat, we sequenced the genomes of 51 strains, isolated from various ecological environments, to understand how E. faecium emerged as a leading hospital pathogen. Because of the scale and diversity of the sampled strains, we were able to resolve the lineage responsible for epidemic, multidrug-resistant human infection from other strains and to measure the evolutionary distances between groups. We found that the epidemic hospital-adapted lineage is rapidly evolving and emerged approximately 75 years ago, concomitant with the introduction of antibiotics, from a population that included the majority of animal strains, and not from human commensal lines. We further found that the lineage that included most strains of animal origin diverged from the main human commensal line approximately 3,000 years ago, a time that corresponds to increasing urbanization of humans, development of hygienic practices, and domestication of animals, which we speculate contributed to their ecological separation. Each bifurcation was accompanied by the acquisition of new metabolic capabilities and colonization traits on mobile elements and the loss of function and genome remodeling associated with mobile element insertion and movement. As a result, diversity within the species, in terms of sequence divergence as well as gene content, spans a range usually associated with speciation.
Enterococci, in particular vancomycin-resistant Enterococcus faecium, recently emerged as a leading cause of hospital-acquired infection worldwide. In this study, we examined genome sequence data to understand the bacterial adaptations that accompanied this transformation from microbes that existed for eons as members of host microbiota. We observed changes in the genomes that paralleled changes in human behavior. An initial bifurcation within the species appears to have occurred at a time that corresponds to the urbanization of humans and domestication of animals, and a more recent bifurcation parallels the introduction of antibiotics in medicine and agriculture. In response to the opportunity to fill niches associated with changes in human activity, a rapidly evolving lineage emerged, a lineage responsible for the vast majority of multidrug-resistant E. faecium infections.
PMCID: PMC3747589  PMID: 23963180
12.  CRISPR-Cas: To take up DNA or not, that is the question 
Cell host & microbe  2012;12(2):125-126.
Historically landmark experiments showed that capsule switching is critical for Streptococcus pneumonia survival. Further studies demonstrated that capsule ‘transformation’ occurs via DNA uptake. In this issue of Cell Host and Microbe, Bikard et al. (2012) show that CRISPR-Cas systems inhibit DNA uptake, selecting for the outgrowth of CRISPR-defective pneumococci.
PMCID: PMC3427645  PMID: 22901532
13.  Bacterial endophthalmitis in the age of outpatient intravitreal therapies and cataract surgeries: Host-microbe interactions in intraocular infection 
Bacterial endophthalmitis is a sight threatening infection of the interior structures of the eye. Incidence in the US has increased in recent years, which appears to be related to procedures being performed on an aging population. The advent of outpatient intravitreal therapy for management of age-related macular degeneration raises yet additional risks. Compounding the problem is the continuing progression of antibiotic resistance. Visual prognosis for endophthalmitis depends on the virulence of the causative organism, the severity of intraocular inflammation, and the timeliness of effective therapy. We review the current understanding of the pathogenesis of bacterial endophthalmitis, highlighting opportunities for the development of improved therapeutics and preventive strategies.
PMCID: PMC3361607  PMID: 22521570
Endophthalmitis; Staphylococcus aureus; coagulase-negative staphylococci; Streptococcus pneumoniae; Enterococcus faecalis; Bacillus
14.  Drosophila Host Model Reveals New Enterococcus faecalis Quorum-Sensing Associated Virulence Factors 
PLoS ONE  2013;8(5):e64740.
Enterococcus faecalis V583 is a vancomycin-resistant clinical isolate which belongs to the hospital-adapted clade, CC2. This strain harbours several factors that have been associated with virulence, including the fsr quorum-sensing regulatory system that is known to control the expression of GelE and SprE proteases. To discriminate between genes directly regulated by Fsr, and those indirectly regulated as the result of protease expression or activity, we compared gene expression in isogenic mutants of V583 variously defective in either Fsr quorum sensing or protease expression. Quorum sensing was artificially induced by addition of the quorum signal, GBAP, exogenously in a controlled manner. The Fsr regulon was found to be restricted to five genes, gelE, sprE, ef1097, ef1351 and ef1352. Twelve additional genes were found to be dependent on the presence of GBAP-induced proteases. Induction of GelE and SprE by GBAP via Fsr resulted in accumulation of mRNA encoding lrgAB, and this induction was found to be lytRS dependent. Drosophila infection was used to discern varying levels of toxicity stemming from mutations in the fsr quorum regulatory system and the genes that it regulates, highlighting the contribution of LrgAB and bacteriocin EF1097 to infection toxicity. A contribution of SprE to infection toxicity was also detected. This work brought to light new players in E. faecalis success as a pathogen and paves the way for future studies on host tolerance mechanisms to infections caused by this important nosocomial pathogen.
PMCID: PMC3667150  PMID: 23734216
15.  Structure, Function, and Biology of the Enterococcus faecalis Cytolysin  
Toxins  2013;5(5):895-911.
Enterococcus faecalis is a Gram-positive commensal member of the gut microbiota of a wide range of organisms. With the advent of antibiotic therapy, it has emerged as a multidrug resistant, hospital-acquired pathogen. Highly virulent strains of E. faecalis express a pore-forming exotoxin, called cytolysin, which lyses both bacterial and eukaryotic cells in response to quorum signals. Originally described in the 1930s, the cytolysin is a member of a large class of lanthionine-containing bacteriocins produced by Gram-positive bacteria. While the cytolysin shares some core features with other lantibiotics, it possesses unique characteristics as well. The current understanding of cytolysin biosynthesis, structure/function relationships, and contribution to the biology of E. faecalis are reviewed, and opportunities for using emerging technologies to advance this understanding are discussed.
PMCID: PMC3709268  PMID: 23628786
cytolysin; lantibiotic; bacteriocin
16.  The socio-cultural importance of Mauritia flexuosa palm swamps (aguajales) and implications for multi-use management in two Maijuna communities of the Peruvian Amazon 
Fruit from the palm Mauritia flexuosa (aguaje) is harvested throughout the Peruvian Amazon for subsistence and commercial purposes. Recent estimates suggest that residents of Iquitos, the largest city in the region, consume approximately 148.8 metric tons of aguaje fruit per month, the vast majority of which is harvested by felling and killing adult female trees. In this study, we sought to better understand and document the importance of M. flexuosa palm swamps (aguajales) in two Maijuna indigenous communities to inform the sustainable management of this habitat and species.
Semi-structured interviews, focus groups, and household surveys were carried out to assess the significance of aguajales and their associated plant and animal resources as well as to determine how the relationship that the Maijuna have with aguajales has changed over time.
Aguajales and their associated resources are culturally significant and useful to the Maijuna in a wide variety of ways. In addition to M. flexuosa, the Maijuna use over 60 different species of plants from aguajales. When M. flexuosa is in fruit, aguajales are important hunting areas with a total of 20 different animal species hunted. The Maijuna also have traditional beliefs about aguajales, believing that malevolent supernatural beings reside in them. Notably, the relationship that the Maijuna have with aguajales has changed considerably over the years as aguaje fruit went from a subsistence item collected opportunistically from the ground to a market good destructively harvested beginning in the early 1990s. The Maijuna are concerned not only about how this has affected the future commercial harvest of aguaje but also about its effects on game animals given the importance of hunting to Maijuna cultural identity, subsistence, and income generation.
In order to meet the multiple socio-cultural and economic needs of the Maijuna, sustainable management efforts must be expanded to not only focus on the commercial harvest of aguaje but also other facets of their relationship with this habitat. Our study suggests that the research and development of multi-use forest management plans must not be restricted to commercial forest products and ecosystem services given that many communities rely on tropical forests for a wide range of non-market cultural, economic, and subsistence goods and services.
PMCID: PMC3733440  PMID: 23607601
Ethnoecology; Multi-use management; Forest resources; Maijuna; Peruvian Amazon; Mauritia flexuosa
17.  Annual Screening Strategies in BRCA1 and BRCA2 Gene Mutation Carriers: A Comparative Effectiveness Analysis 
Cancer  2011;118(8):2021-2030.
While breast cancer screening with mammography and MRI is recommended for BRCA mutation carriers, there is no current consensus on the optimal screening regimen.
We used a computer simulation model to compare six annual screening strategies [film mammography (FM), digital mammography (DM), FM and magnetic resonance imaging (MRI) or DM and MRI contemporaneously, and alternating FM/MRI or DM/MRI at six-month intervals] beginning at ages 25, 30, 35, and 40, and two strategies of annual MRI with delayed alternating DM/FM to clinical surveillance alone. Strategies were evaluated without and with mammography-induced breast cancer risk, using two models of excess relative risk. Input parameters were obtained from the medical literature, publicly available databases, and calibration.
Without radiation risk effects, alternating DM/MRI starting at age 25 provided the highest life expectancy (BRCA1: 72.52 years, BRCA2: 77.63 years). When radiation risk was included, a small proportion of diagnosed cancers were attributable to radiation exposure (BRCA1: <2%, BRCA2: <4%). With radiation risk, alternating DM/MRI at age 25 or annual MRI at age 25/delayed alternating DM at age 30 were most effective, depending on the radiation risk model used. Alternating DM/MRI starting at age 25 also had the highest number of false-positive screens/person (BRCA1: 4.5, BRCA2: 8.1).
Annual MRI at 25/delayed alternating DM at age 30 is likely the most effective screening strategy in BRCA mutation carriers. Screening benefits, associated risks and personal acceptance of false-positive results, should be considered in choosing the optimal screening strategy for individual women.
PMCID: PMC3245774  PMID: 21935911
BRCA1 gene; BRCA2 gene; Breast neoplasms; Mass screening; Computer simulation
18.  Photodynamic and Antibiotic Therapy Impair the Pathogenesis of Enterococcus faecium in a Whole Animal Insect Model 
PLoS ONE  2013;8(2):e55926.
Enterococcus faecium has emerged as one of the most important pathogens in healthcare-associated infections worldwide due to its intrinsic and acquired resistance to many antibiotics, including vancomycin. Antimicrobial photodynamic therapy (aPDT) is an alternative therapeutic platform that is currently under investigation for the control and treatment of infections. PDT is based on the use of photoactive dye molecules, widely known as photosensitizer (PS). PS, upon irradiation with visible light, produces reactive oxygen species that can destroy lipids and proteins causing cell death. We employed Galleria mellonella (the greater wax moth) caterpillar fatally infected with E. faecium to develop an invertebrate host model system that can be used to study the antimicrobial PDT (alone or combined with antibiotics). In the establishment of infection by E. faecium in G. mellonella, we found that the G. mellonella death rate was dependent on the number of bacterial cells injected into the insect hemocoel and all E. faecium strains tested were capable of infecting and killing G. mellonella. Antibiotic treatment with ampicillin, gentamicin or the combination of ampicillin and gentamicin prolonged caterpillar survival infected by E. faecium (P = 0.0003, P = 0.0001 and P = 0.0001, respectively). In the study of antimicrobial PDT, we verified that methylene blue (MB) injected into the insect followed by whole body illumination prolonged the caterpillar survival (P = 0.0192). Interestingly, combination therapy of larvae infected with vancomycin-resistant E. faecium, with antimicrobial PDT followed by vancomycin, significantly prolonged the survival of the caterpillars when compared to either antimicrobial PDT (P = 0.0095) or vancomycin treatment alone (P = 0.0025), suggesting that the aPDT made the vancomycin resistant E. faecium strain more susceptible to vancomycin action. In summary, G. mellonella provides an invertebrate model host to study the antimicrobial PDT and to explore combinatorial aPDT-based treatments.
PMCID: PMC3573038  PMID: 23457486
19.  Viral Diversity Threshold for Adaptive Immunity in Prokaryotes 
mBio  2012;3(6):e00456-12.
Bacteria and archaea face continual onslaughts of rapidly diversifying viruses and plasmids. Many prokaryotes maintain adaptive immune systems known as clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated genes (Cas). CRISPR-Cas systems are genomic sensors that serially acquire viral and plasmid DNA fragments (spacers) that are utilized to target and cleave matching viral and plasmid DNA in subsequent genomic invasions, offering critical immunological memory. Only 50% of sequenced bacteria possess CRISPR-Cas immunity, in contrast to over 90% of sequenced archaea. To probe why half of bacteria lack CRISPR-Cas immunity, we combined comparative genomics and mathematical modeling. Analysis of hundreds of diverse prokaryotic genomes shows that CRISPR-Cas systems are substantially more prevalent in thermophiles than in mesophiles. With sequenced bacteria disproportionately mesophilic and sequenced archaea mostly thermophilic, the presence of CRISPR-Cas appears to depend more on environmental temperature than on bacterial-archaeal taxonomy. Mutation rates are typically severalfold higher in mesophilic prokaryotes than in thermophilic prokaryotes. To quantitatively test whether accelerated viral mutation leads microbes to lose CRISPR-Cas systems, we developed a stochastic model of virus-CRISPR coevolution. The model competes CRISPR-Cas-positive (CRISPR-Cas+) prokaryotes against CRISPR-Cas-negative (CRISPR-Cas−) prokaryotes, continually weighing the antiviral benefits conferred by CRISPR-Cas immunity against its fitness costs. Tracking this cost-benefit analysis across parameter space reveals viral mutation rate thresholds beyond which CRISPR-Cas cannot provide sufficient immunity and is purged from host populations. These results offer a simple, testable viral diversity hypothesis to explain why mesophilic bacteria disproportionately lack CRISPR-Cas immunity. More generally, fundamental limits on the adaptability of biological sensors (Lamarckian evolution) are predicted.
A remarkable recent discovery in microbiology is that bacteria and archaea possess systems conferring immunological memory and adaptive immunity. Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated genes (CRISPR-Cas) are genomic sensors that allow prokaryotes to acquire DNA fragments from invading viruses and plasmids. Providing immunological memory, these stored fragments destroy matching DNA in future viral and plasmid invasions. CRISPR-Cas systems also provide adaptive immunity, keeping up with mutating viruses and plasmids by continually acquiring new DNA fragments. Surprisingly, less than 50% of mesophilic bacteria, in contrast to almost 90% of thermophilic bacteria and Archaea, maintain CRISPR-Cas immunity. Using mathematical modeling, we probe this dichotomy, showing how increased viral mutation rates can explain the reduced prevalence of CRISPR-Cas systems in mesophiles. Rapidly mutating viruses outrun CRISPR-Cas immune systems, likely decreasing their prevalence in bacterial populations. Thus, viral adaptability may select against, rather than for, immune adaptability in prokaryotes.
PMCID: PMC3517865  PMID: 23221803
20.  Biocompatibility and biofilm inhibition of N,N-hexyl,methyl-polyethylenimine bonded to Boston Keratoprosthesis materials 
Biomaterials  2011;32(34):8783-8796.
The biocompatibility and antibacterial properties of N,N-hexyl,methyl-polyethylenimine (HMPEI) covalently attached to the Boston Keratoprosthesis (B-KPro) materials was evaluated. By means of confocal and electron microscopies, we observed that HMPEI-derivatized materials exert an inhibitory effect on biofilm formation by Staphylococcus aureus clinical isolates, as compared to the parent poly(methyl methacrylate) (PMMA) and titanium. There was no additional corneal epithelial cell cytotoxicity of HMPEI-coated PMMA compared to that of control PMMA in tissue cultures in vitro. Likewise, no toxicity or adverse reactivity was detected with HMPEI-derivatized PMMA or titanium compared to those of the control materials after intrastromal or anterior chamber implantation in rabbits in vivo.
PMCID: PMC3410931  PMID: 21903257
antibacterial; polyethylenimine (PEI); keratoprosthesis; PMMA; titanium; Staphylococcus aureus; corneal cytotoxicity
21.  An Antibiotic That Inhibits a Late Step in Wall Teichoic Acid Biosynthesis Induces the Cell Wall Stress Stimulon in Staphylococcus aureus 
Wall teichoic acids (WTAs) are phosphate-rich, sugar-based polymers attached to the cell walls of most Gram-positive bacteria. In Staphylococcus aureus, these anionic polymers regulate cell division, protect cells from osmotic stress, mediate host colonization, and mask enzymatically susceptible peptidoglycan bonds. Although WTAs are not required for survival in vitro, blocking the pathway at a late stage of synthesis is lethal. We recently discovered a novel antibiotic, targocil, that inhibits a late acting step in the WTA pathway. Its target is TarG, the transmembrane component of the ABC transporter (TarGH) that exports WTAs to the cell surface. We examined here the effects of targocil on S. aureus using transmission electron microscopy and gene expression profiling. We report that targocil treatment leads to multicellular clusters containing swollen cells displaying evidence of osmotic stress, strongly induces the cell wall stress stimulon, and reduces the expression of key virulence genes, including dltABCD and capsule genes. We conclude that WTA inhibitors that act at a late stage of the biosynthetic pathway may be useful as antibiotics, and we present evidence that they could be particularly useful in combination with beta-lactams.
PMCID: PMC3318382  PMID: 22290958
22.  Comparative Genomics of Vancomycin-Resistant Staphylococcus aureus Strains and Their Positions within the Clade Most Commonly Associated with Methicillin-Resistant S. aureus Hospital-Acquired Infection in the United States  
mBio  2012;3(3):e00112-12.
Methicillin-resistant Staphylococcus aureus (MRSA) strains are leading causes of hospital-acquired infections in the United States, and clonal cluster 5 (CC5) is the predominant lineage responsible for these infections. Since 2002, there have been 12 cases of vancomycin-resistant S. aureus (VRSA) infection in the United States—all CC5 strains. To understand this genetic background and what distinguishes it from other lineages, we generated and analyzed high-quality draft genome sequences for all available VRSA strains. Sequence comparisons show unambiguously that each strain independently acquired Tn1546 and that all VRSA strains last shared a common ancestor over 50 years ago, well before the occurrence of vancomycin resistance in this species. In contrast to existing hypotheses on what predisposes this lineage to acquire Tn1546, the barrier posed by restriction systems appears to be intact in most VRSA strains. However, VRSA (and other CC5) strains were found to possess a constellation of traits that appears to be optimized for proliferation in precisely the types of polymicrobic infection where transfer could occur. They lack a bacteriocin operon that would be predicted to limit the occurrence of non-CC5 strains in mixed infection and harbor a cluster of unique superantigens and lipoproteins to confound host immunity. A frameshift in dprA, which in other microbes influences uptake of foreign DNA, may also make this lineage conducive to foreign DNA acquisition.
Invasive methicillin-resistant Staphylococcus aureus (MRSA) infection now ranks among the leading causes of death in the United States. Vancomycin is a key last-line bactericidal drug for treating these infections. However, since 2002, vancomycin resistance has entered this species. Of the now 12 cases of vancomycin-resistant S. aureus (VRSA), each was believed to represent a new acquisition of the vancomycin-resistant transposon Tn1546 from enterococcal donors. All acquisitions of Tn1546 so far have occurred in MRSA strains of the clonal cluster 5 genetic background, the most common hospital lineage causing hospital-acquired MRSA infection. To understand the nature of these strains, we determined and examined the nucleotide sequences of the genomes of all available VRSA. Genome comparison identified candidate features that position strains of this lineage well for acquiring resistance to antibiotics in mixed infection.
PMCID: PMC3372964  PMID: 22617140
23.  Persisting Viral Sequences Shape Microbial CRISPR-based Immunity 
PLoS Computational Biology  2012;8(4):e1002475.
Well-studied innate immune systems exist throughout bacteria and archaea, but a more recently discovered genomic locus may offer prokaryotes surprising immunological adaptability. Mediated by a cassette-like genomic locus termed Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), the microbial adaptive immune system differs from its eukaryotic immune analogues by incorporating new immunities unidirectionally. CRISPR thus stores genomically recoverable timelines of virus-host coevolution in natural organisms refractory to laboratory cultivation. Here we combined a population genetic mathematical model of CRISPR-virus coevolution with six years of metagenomic sequencing to link the recoverable genomic dynamics of CRISPR loci to the unknown population dynamics of virus and host in natural communities. Metagenomic reconstructions in an acid-mine drainage system document CRISPR loci conserving ancestral immune elements to the base-pair across thousands of microbial generations. This ‘trailer-end conservation’ occurs despite rapid viral mutation and despite rapid prokaryotic genomic deletion. The trailer-ends of many reconstructed CRISPR loci are also largely identical across a population. ‘Trailer-end clonality’ occurs despite predictions of host immunological diversity due to negative frequency dependent selection (kill the winner dynamics). Statistical clustering and model simulations explain this lack of diversity by capturing rapid selective sweeps by highly immune CRISPR lineages. Potentially explaining ‘trailer-end conservation,’ we record the first example of a viral bloom overwhelming a CRISPR system. The polyclonal viruses bloom even though they share sequences previously targeted by host CRISPR loci. Simulations show how increasing random genomic deletions in CRISPR loci purges immunological controls on long-lived viral sequences, allowing polyclonal viruses to bloom and depressing host fitness. Our results thus link documented patterns of genomic conservation in CRISPR loci to an evolutionary advantage against persistent viruses. By maintaining old immunities, selection may be tuning CRISPR-mediated immunity against viruses reemerging from lysogeny or migration.
Author Summary
Most microbes appear unculturable in the laboratory, limiting our knowledge of how virus and prokaryotic host evolve in natural systems. However, a genomic locus found in many prokaryotes, CRISPR, may offer cultivation-independent probes of virus-microbe coevolution. Utilizing nearby genes, CRISPR can serially incorporate short viral and plasmid sequences. These sequences bind and cleave cognate regions in subsequent viral and plasmid insertions, conferring adaptive anti-viral and anti-plasmid immunity. By incorporating sequences undirectionally, CRISPR also provides timelines of virus-prokaryote coevolution. Yet, CRISPR only incorporates 30–80 base-pair viral sequences, leaving incomplete coevolutionary recordings. To reconstruct the missing coevolutionary dynamics shaping natural CRISPRs, we combined metagenomic reconstructions with population-scale mathematical modeling. Capturing rare and rapid sweeps of CRISPR diversity by highly immune lines, mathematical modeling explains why naturally reconstructed CRISPR loci are often largely identical across a population. Both model and experiment further document surprising proliferations of old viral sequences against which hosts had preexisting CRISPR immunity. Due to these deadly blooms of ancestral viral elements, CRISPR's conservation of old immune sequences appears to confer a selective advantage. This may explain the striking immunological memory documented in CRISPR loci, which occurs despite rapid viral mutation and despite rapid deletions in prokaryotic genomes.
PMCID: PMC3330103  PMID: 22532794
24.  A Metalloproteinase Secreted by Streptococcus pneumoniae Removes Membrane Mucin MUC16 from the Epithelial Glycocalyx Barrier 
PLoS ONE  2012;7(3):e32418.
The majority of bacterial infections occur across wet-surfaced mucosal epithelia, including those that cover the eye, respiratory tract, gastrointestinal tract and genitourinary tract. The apical surface of all these mucosal epithelia is covered by a heavily glycosylated glycocalyx, a major component of which are membrane-associated mucins (MAMs). MAMs form a barrier that serves as one of the first lines of defense against invading bacteria. While opportunistic bacteria rely on pre-existing defects or wounds to gain entry to epithelia, non opportunistic bacteria, especially the epidemic disease-causing ones, gain access to epithelial cells without evidence of predisposing injury. The molecular mechanisms employed by these non opportunistic pathogens to breach the MAM barrier remain unknown. To test the hypothesis that disease-causing non opportunistic bacteria gain access to the epithelium by removal of MAMs, corneal, conjunctival, and tracheobronchial epithelial cells, cultured to differentiate to express the MAMs, MUCs 1, 4, and 16, were exposed to a non encapsulated, non typeable strain of Streptococcus pneumoniae (SP168), which causes epidemic conjunctivitis. The ability of strain SP168 to induce MAM ectodomain release from epithelia was compared to that of other strains of S. pneumoniae, as well as the opportunistic pathogen Staphylococcus aureus. The experiments reported herein demonstrate that the epidemic disease-causing S. pneumoniae species secretes a metalloproteinase, ZmpC, which selectively induces ectodomain shedding of the MAM MUC16. Furthermore, ZmpC-induced removal of MUC16 from the epithelium leads to loss of the glycocalyx barrier function and enhanced internalization of the bacterium. These data suggest that removal of MAMs by bacterial enzymes may be an important virulence mechanism employed by disease-causing non opportunistic bacteria to gain access to epithelial cells to cause infection.
PMCID: PMC3296694  PMID: 22412870
25.  Comparative Genomics of Enterococci: Variation in Enterococcus faecalis, Clade Structure in E. faecium, and Defining Characteristics of E. gallinarum and E. casseliflavus 
mBio  2012;3(1):e00318-11.
The enterococci are Gram-positive lactic acid bacteria that inhabit the gastrointestinal tracts of diverse hosts. However, Enterococcus faecium and E. faecalis have emerged as leading causes of multidrug-resistant hospital-acquired infections. The mechanism by which a well-adapted commensal evolved into a hospital pathogen is poorly understood. In this study, we examined high-quality draft genome data for evidence of key events in the evolution of the leading causes of enterococcal infections, including E. faecalis, E. faecium, E. casseliflavus, and E. gallinarum. We characterized two clades within what is currently classified as E. faecium and identified traits characteristic of each, including variation in operons for cell wall carbohydrate and putative capsule biosynthesis. We examined the extent of recombination between the two E. faecium clades and identified two strains with mosaic genomes. We determined the underlying genetics for the defining characteristics of the motile enterococci E. casseliflavus and E. gallinarum. Further, we identified species-specific traits that could be used to advance the detection of medically relevant enterococci and their identification to the species level.
The enterococci, in particular, vancomycin-resistant enterococci, have emerged as leading causes of multidrug-resistant hospital-acquired infections. In this study, we examined genome sequence data to define traits with the potential to influence host-microbe interactions and to identify sequences and biochemical functions that could form the basis for the rapid identification of enterococcal species or lineages of importance in clinical and environmental samples.
PMCID: PMC3374389  PMID: 22354958

Results 1-25 (67)