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1.  Irreproducible and Uninterpretable Polymyxin B MICs for Enterobacter cloacae and Enterobacter aerogenes 
Journal of Clinical Microbiology  2013;51(12):4106-4111.
Carbapenem-resistant Enterobacter species are emerging nosocomial pathogens. As with most multidrug-resistant Gram-negative pathogens, the polymyxins are often the only therapeutic option. In this study involving clinical isolates of E. cloacae and E. aerogenes, susceptibility testing methods with polymyxin B were analyzed. All isolates underwent testing by the broth microdilution (in duplicate) and agar dilution (in duplicate) methods, and select isolates were examined by the Etest method. Selected isolates were also examined for heteroresistance by population analysis profiling. Using a susceptibility breakpoint of ≤2 μg/ml, categorical agreement by all four dilution tests (two broth microdilution and two agar dilution) was achieved in only 76/114 (67%) of E. cloacae isolates (65 susceptible, 11 resistant). Thirty-eight (33%) had either conflicting or uninterpretable results (multiple skip wells, i.e., wells that exhibit no growth although growth does occur at higher concentrations). Of the 11 consistently resistant isolates, five had susceptible MICs as determined by Etest. Heteroresistant subpopulations were detected in eight of eight isolates tested, with greater percentages in isolates with uninterpretable MICs. For E. aerogenes, categorical agreement between the four dilution tests was obtained in 48/56 (86%), with conflicting and/or uninterpretable results in 8/56 (14%). For polymyxin susceptibility testing of Enterobacter species, close attention must be paid to the presence of multiple skip wells, leading to uninterpretable results. Susceptibility also should not be assumed based on the results of a single test. Until the clinical relevance of skip wells is defined, interpretation of polymyxin susceptibility tests for Enterobacter species should be undertaken with extreme caution.
PMCID: PMC3838098  PMID: 24088860
2.  Accuracy of Carbapenem Nonsusceptibility for Identification of KPC-Possessing Enterobacteriaceae by Use of the Revised CLSI Breakpoints ▿  
Journal of Clinical Microbiology  2011;49(11):3931-3933.
Using the updated 2010 CLSI carbapenem breakpoints for the Enterobacteriaceae, nonsusceptibility to ertapenem and imipenem predicted the presence of blaKPC poorly, especially among Escherichia coli and Enterobacter species. In regions where KPC-producing bacteria are endemic, testing for nonsusceptibility to meropenem may provide improved accuracy in identifying these isolates.
PMCID: PMC3209089  PMID: 21880962
3.  Susceptibility Profiles, Molecular Epidemiology, and Detection of KPC-Producing Escherichia coli Isolates from the New York City Vicinity ▿  
Journal of Clinical Microbiology  2010;48(12):4604-4607.
The detection of Enterobacteriaceae carrying the carbapenemase KPC has been problematic. Thirty isolates of KPC-possessing Escherichia coli were gathered from hospitals in New York City and Connecticut. The imipenem, meropenem, doripenem, and ertapenem MIC50 values were 4, 2, 1, and 4 μg/ml, respectively. Over half of the isolates belonged to a single ribotype. Using an ertapenem breakpoint of 0.25 μg/ml would efficiently detect these isolates.
PMCID: PMC3008498  PMID: 20926706
4.  Contribution of OmpK36 to carbapenem susceptibility in KPC-producing Klebsiella pneumoniae 
Journal of Medical Microbiology  2009;58(Pt 10):1303-1308.
Isolates of Klebsiella pneumoniae harbouring the carbapenemase KPC may have carbapenem MICs that remain in the susceptible range, and may therefore go unrecognized. To understand the mechanisms contributing to the variability in carbapenem MICs, 20 clinical isolates, all belonging to either of two clonal groups of KPC-possessing K. pneumoniae endemic to New York City, were examined. Expression of genes encoding KPC, the porins OmpK35 and OmpK36, and the efflux pump AcrAB was examined by real-time RT-PCR. Outer-membrane profiles of selected KPC-producing isolates were examined by SDS-PAGE, and proteins were identified by matrix-assisted laser desorption/ionization mass spectrometry. The identification of SHV and TEM β-lactamases and the genomic sequences of ompK35 and ompK36 were determined by PCR and DNA sequencing, respectively. For one clonal group, carbapenem MICs increased with decreasing expression of ompK36. A second clonal group also had carbapenem MICs that correlated with ompK36 expression. However, all of the isolates in this latter group continued to produce OmpK36, suggesting that porin configuration may affect entry of carbapenems. For isolates that had the greatest expression of ompK36, carbapenem MICs tended to be lower when determined by the broth microdilution technique, and scattered colonies were seen around the Etest zones of inhibition. All of the KPC-producing isolates were highly resistant to ertapenem, regardless of ompK36 expression. In conclusion, isolates of KPC-possessing K. pneumoniae that express ompK36 tend to have lower MICs to carbapenems and therefore may be more difficult to detect by clinical laboratories. Regardless of ompK36 expression, all of the KPC producers were consistently resistant to ertapenem.
PMCID: PMC2887543  PMID: 19556371
5.  Correlation of the expression of acrB and the regulatory genes marA, soxS and ramA with antimicrobial resistance in clinical isolates of Klebsiella pneumoniae endemic to New York City 
Nosocomial isolates of Klebsiella pneumoniae resistant to all commonly used antimicrobial agents have emerged in many regions of the world. It is unknown if efflux systems contribute to the multidrug resistance phenotype.
The expression of genes encoding the efflux pump AcrAB and the global regulators MarA, SoxS and RamA were examined and correlated with antimicrobial resistance.
Twenty isolates belonged to the two important clones representing KPC-possessing strains endemic to our region. Virtually all of these isolates had negligible or absent expression of the genes, and resistance to fluoroquinolones and aminoglycosides could be explained by alternative mechanisms. All of these isolates were susceptible to tigecycline. A group of 14 heterogeneous isolates was also examined. There was a correlation between expression of marA with expression of soxS. Only expression of soxS was significantly correlated with expression of acrB. With a background substitution in GyrA, increased expression of acrB and marA appeared to contribute to fluoroquinolone resistance in some isolates. A correlation was noted between expression of soxS and ramA (but not marA and acrB) and tigecycline MICs. Following in vitro exposure to tigecycline, resistance occurred in association with a marked increase in marA and acrB expression in isolates lacking expression of soxS and ramA.
While laboratory-derived tigecycline resistance was associated with increased acrB expression, the variation in tigecycline MICs in clinical isolates was associated only with selected regulator genes. It appears that other mechanisms beyond activation of the acrAB system mediate tigecycline resistance.
PMCID: PMC2707265  PMID: 19457933
efflux; tigecycline; multidrug-resistant
6.  Polymyxins Revisited 
Clinical Microbiology Reviews  2008;21(3):449-465.
The global emergence of multidrug-resistant gram-negative bacilli has spurred a renewed interest in polymyxins. Once discarded due to concerns regarding nephrotoxicity and neurotoxicity, polymyxins now hold an important role in the antibiotic armamentarium. However, more reliable information is needed to determine the optimal dosing of these agents. Also, unanswered questions regarding in vitro testing remain, including questions regarding the reliability of automated systems and the establishment of appropriate breakpoints for defining susceptibility. Most contemporary clinical studies examining the use of these agents have involved patients with infections due to multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii strains. It has been reassuring that polymyxin therapy for resistant bacteria has resulted in clinical responses and toxicity rates similar to those for carbapenem therapy for susceptible isolates. While most surveillance studies demonstrated high rates of susceptibility, several reports noted the emergence of polymyxin-resistant nosocomial pathogens. Polymyxins have assumed an important antibiotic niche for therapy for hospital-acquired infections; further studies defining the optimal use of these agents will likely extend the duration of their clinical usefulness.
PMCID: PMC2493081  PMID: 18625681
7.  Correlation of Antimicrobial Resistance with β-Lactamases, the OmpA-Like Porin, and Efflux Pumps in Clinical Isolates of Acinetobacter baumannii Endemic to New York City▿  
Acinetobacter baumannii strains resistant to all β-lactams, aminoglycosides, and fluoroquinolones have emerged in many medical centers. Potential mechanisms contributing to antimicrobial resistance were investigated in 40 clinical isolates endemic to New York City. The isolates were examined for the presence of various β-lactamases, aminoglycoside-modifying enzymes, and mutations in gyrA and parC. Expression of the genes encoding the β-lactamase AmpC, the efflux systems AdeABC and AbeM, and the OmpA-like porin was also examined by real-time reverse transcription-PCR. No VIM, IMP, KPC, OXA-23-type, OXA-24-type, or OXA-58 β-lactamases were detected, although several isolates had acquired blaSHV-5. Most cephalosporin-resistant isolates had increased levels of expression of ampC and/or had acquired blaSHV-5; however, isolates without these features still had reduced susceptibility to cefepime that was mediated by the AdeABC efflux system. Although most isolates with ISAba1 upstream of the blaOXA-51-like carbapenemase gene were resistant to meropenem, several remained susceptible to imipenem. The presence of aminoglycoside-modifying enzymes and gyrase mutations accounted for aminoglycoside and fluoroquinolone resistance, respectively. The increased expression of adeABC was not an important contributor to aminoglycoside or fluoroquinolone resistance but did correlate with reduced susceptibility to tigecycline. The expression of abeM and ompA and phenotypic changes in OmpA did not correlate with antimicrobial resistance. A. baumannii has become a well-equipped nosocomial pathogen; defining the relative contribution of these and other mechanisms of antimicrobial resistance will require further investigation.
PMCID: PMC2533509  PMID: 18591275
8.  A population-based study examining the emergence of community-associated methicillin-resistant Staphylococcus aureus USA300 in New York City 
Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is a serious pathogen in several regions in the United States. It is unclear which populations are at high risk for the emergence of these strains.
All unique patient isolates of S. aureus were collected from hospitals in Brooklyn, NY over a three-month period. Isolates of MRSA that were susceptible to clindamycin underwent SCCmec typing. Isolates with the SCCmec type IV (characteristic of CA-MRSA strains) underwent ribotyping. Demographic information involving the neighborhoods of Brooklyn was also gathered and correlated with the prevalence of CA-MRSA strains.
Of 1316 isolates collected during the surveillance, 217 were MRSA susceptible to clindamycin. A total of 125 isolates possessed SCCmec type IV; 72 belonged to the USA300 strain and five belonged to the USA400 strain. Hospitals in the eastern part of the city had the highest prevalence of USA300 strain. Individuals in the eastern region, when compared to the western region, were more likely to be Black, Hispanic, female, and < 18 years of age, and to have households of ≥ 3 persons. In addition, the median household income was lower, and the proportion of individuals on public assistance was higher, for the population in the eastern region.
The USA300 strain of CA-MRSA is emerging in New York City. In this population-based study, urban regions of lower socioeconomic status and with evidence of overcrowding appear to be at higher risk for the emergence of this pathogen.
PMCID: PMC1693566  PMID: 17137512
9.  Interplay of Efflux System, ampC, and oprD Expression in Carbapenem Resistance of Pseudomonas aeruginosa Clinical Isolates 
Carbapenems are important agents for the therapy of infections due to multidrug-resistant Pseudomonas aeruginosa; the development of carbapenem resistance hampers effective therapeutic options. To assess the mechanisms leading to resistance, 33 clinical isolates with differing degrees of carbapenem susceptibility were analyzed for the expression of the chromosomal β-lactamase (ampC), the porin that is important for the entry of carbapenems (oprD), and the proteins involved in four efflux systems (mexA, mexC, mexE, and mexX). Real-time reverse transcriptase PCR was performed using primers and fluorescent probes for each of the target genes. The sequencing of regulatory genes (ampR, mexR, nalC, nalD, mexT, and mexZ) was also performed. Diminished expression of oprD was present in all imipenem- and meropenem-resistant isolates but was not required for ertapenem resistance. Increased expression of ampC was not observed in several isolates that were overtly resistant to carbapenems. Increased expression of several efflux systems was observed in many of the carbapenem-resistant isolates. Increased efflux activity correlated with high-level ertapenem resistance and reduced susceptibility to meropenem and aztreonam. Most isolates with increased expression of mexA had mutations affecting nalC and/or nalD. Two isolates with mutations leading to a premature stop codon in mexZ had markedly elevated mexX expressions, although mutations in mexZ were not a prerequisite for overexpression. β-Lactam resistance in clinical isolates of P. aeruginosa is a result of the interplay between diminished production of oprD, increased activity of ampC, and several efflux systems.
PMCID: PMC1472219  PMID: 16641429
10.  Emergence of KPC-Possessing Klebsiella pneumoniae in Brooklyn, New York: Epidemiology and Recommendations for Detection 
Among 257 isolates of Klebsiella pneumoniae collected in Brooklyn, NY, 24% were found to possess blaKPC. Clinical microbiology laboratories that used automated broth microdilution systems reported 15% of the KPC-possessing isolates as susceptible to imipenem. The imipenem MIC was found to be markedly affected by the inoculum. For accurate detection of KPC-possessing K. pneumoniae, particular attention should be paid to proper inoculum preparation for broth-based susceptibility methods. In addition, using ertapenem or meropenem for class reporting of carbapenem susceptibility will improve detection.
PMCID: PMC1168646  PMID: 15980389
11.  Community-associated Methicillin-resistant Staphylococcus aureus in Hospital Nursery and Maternity Units 
Emerging Infectious Diseases  2005;11(6):808-813.
Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has rarely been reported in the hospital setting. We report an outbreak of 7 cases of skin and soft tissue infections due to a strain of CA-MRSA. All patients were admitted to the labor and delivery, nursery, or maternity units during a 3-week period. Genetic fingerprinting showed that the outbreak strain was closely related to the USA 400 strain that includes the midwestern strain MW2. All isolates contained the staphylococcal chromosome cassette mec type IV. Genes for Panton-Valentine leukocidin and staphylococcal enterotoxin K were detected in all isolates, and most contained other enterotoxin genes. Testing of nearly 2,000 MRSA isolates collected during citywide surveillance studies from 1999 to 2003 showed that ≈1% were genetically related to MW2. CA-MRSA strain MW2 has been present in this region at least since 1999. This study documents the spread of this strain among healthy newborns at 1 hospital.
PMCID: PMC3367583  PMID: 15963273
Keywords: infection control; Methicillin Resistance; Staphylococcus aureus
12.  Detection of KPC Carbapenem-Hydrolyzing Enzymes in Enterobacter spp. from Brooklyn, New York 
Enterobacter spp. are rarely resistant to carbapenems. In this report, one Enterobacter sp. isolate possessed blaKPC-3 and two possessed blaKPC-2. For all three strains, the imipenem MICs were dependent on the inoculum and testing method; two were reported by the clinical laboratories to be carbapenem susceptible. Improved detection methods will be necessary to identify these enzymes.
PMCID: PMC547228  PMID: 15673765
13.  Streptococcus pneumoniae, Brooklyn, New York: Fluoroquinolone Resistance at our Doorstep 
Emerging Infectious Diseases  2002;8(6):594-597.
To examine the resistance rates and epidemiology of Streptococcus pneumoniae in Brooklyn, New York, isolates were collected during two boroughwide surveillance periods in 1997 and 1999. Of 138 isolates, 67% were susceptible to penicillin and 34% to ciprofloxacin. Susceptibility rates to ciprofloxacin decreased dramatically from 1997 to 1999 (47% to 16%, p=0.0003). Five isolates (3.6%) were resistant to levofloxacin. Western Brooklyn had lower rates of susceptibility to penicillin compared with eastern neighborhoods. More isolates in the eastern neighborhoods belonged to the Spanish/French 9/14 clone, and isolates in the western neighborhoods tended to belong to the Spanish/USA 23F clone. Residents of the western neighborhoods were more likely to be white and elderly and less likely to be receiving Medicaid or public assistance, characteristics associated with increased health-care and antibiotic use. Brooklyn residents appear to be at high risk for fluoroquinolone-resistant S. pneumoniae. Our results underscore the need for vigilant regional surveillance.
PMCID: PMC2738490  PMID: 12023915
Streptococcus pneumoniae; microbial drug resistance; fluoroquinolone anti-infective agents
14.  Comparison of Automated Ribotyping to Pulsed-Field Gel Electrophoresis for Genetic Fingerprinting of Streptococcus pneumoniae 
Journal of Clinical Microbiology  2001;39(11):4175-4177.
Fifty-two isolates of Streptococcus pneumoniae were characterized by pulsed-field gel electrophoresis (PFGE) and automated ribotyping by using HindIII and PvuII. HindIII ribotypes correlated well with PFGE. PvuII produced fewer bands and was less discriminatory. Automated ribotyping with HindIII is an accurate method for genetic fingerprinting of S. pneumoniae and can complement PFGE.
PMCID: PMC88510  PMID: 11682553

Results 1-14 (14)