title: A comparative evaluation of phenotypic and molecular methods for the detection of oxacillin resistance in coagulase-negative staphylococci.
abstract: Detection of oxacillin resistance in coagulase-negative staphylococci (C-NS) by phenotypic methods is often difficult. The present study compared the National Committee for Clinical Laboratory Standards (NCCLS) revised guidelines of phenotypic methods with a mecA-based polymerase chain reaction (PCR) for C-NS. Ninety clinical C-NS isolates were tested for oxacillin resistance by disk diffusion (1-microg disk), minimum inhibitory concentration (MIC) breakpoint (0.5 microg/ml) after 24 h, and mecA-based PCR. The sensitivity and specificity of disk diffusion was 80% and 93%, and the sensitivity and specificity of the MIC breakpoint after 24 h was 84% and 91%, respectively, against PCR as gold standard. Eleven strains (7 mecA-positive and 4 mecA-negative) showed discordant results between MIC breakpoint after 24 h and PCR. Six of the 7 mecA-positive and all 4 mecA-negative discordant strains had inducible oxacillin resistance and beta-lactamase hyperproduction, respectively. The present study concludes that inducible oxacillin resistance and beta-lactamase hyperproduction are the major causes of discordant results between phenotypic methods and mecA-based PCR, and need special attention.
authors.affiliation: Department of Microbiology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow 226 014, India.
authors.forename: Ujjala
authors.initials: U
authors.lastname: Ghoshal
authors.forename: Kashi N
authors.initials: KN
authors.lastname: Prasad
authors.forename: Manisha
authors.initials: M
authors.lastname: Singh
authors.forename: Dinesh P
authors.initials: DP
authors.lastname: Tiwari
authors.forename: Archana
authors.initials: A
authors.lastname: Ayyagari
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003030:Coagulase / Q000378:metabolism*; D006801:Humans; D016106:Methicillin Resistance / Q000235:genetics*; D008826:Microbial Sensitivity Tests / Q000379:methods / Q000592:standards; D002267:Muramoylpentapeptide Carboxypeptidase / Q000235:genetics; D010068:Oxacillin / Q000494:pharmacology*; D010641:Phenotype; D016133:Polymerase Chain Reaction / Q000379:methods*; D012680:Sensitivity and Specificity; D013203:Staphylococcal Infections / Q000382:microbiology; D013210:Staphylococcus / Q000187:drug effects* / Q000201:enzymology
chemical_list: D000900:Anti-Bacterial Agents; D003030:Coagulase; D002267:Muramoylpentapeptide Carboxypeptidase; D010068:Oxacillin