title: Detection and expression of methicillin/oxacillin resistance in multidrug-resistant and non-multidrug-resistant Staphylococcus aureus in Central Sydney, Australia.
abstract: Ninety clinical Staphylococcus aureus isolates from separate patients were examined phenotypically and genotypically for susceptibility to methicillin/oxacillin. Thirty were methicillin/oxacillin susceptible and 60 were methicillin and oxacillin resistant (MRSA). The 60 MRSA isolates examined were subdivided into two groups according to their antibiotic profiles and comprised 30 non-multidrug-resistant (NMDR) isolates, resistant to less than two non-beta-lactam antibiotics, and 30 multidrug-resistant (MDR) isolates, resistant to three or more non-beta-lactam antibiotics. Phenotypic and genotypic analysis of methicillin/oxacillin showed that despite use of the guidelines published by the NCCLS for the testing of S. aureus susceptibility to methicillin/oxacillin, MIC values of some NMDR MRSA isolates fell below the NCCLS-recommended breakpoints. Etest strips failed to detect two NMDR MRSA isolates tested with oxacillin and four tested with methicillin. Lowering the NCCLS-recommended oxacillin screen agar concentration from 6 to 2 mg/L and temperature of incubation to 30 degrees C, improved the specificity and sensitivity of NMDR MRSA detection from 87% to 100%. On PFGE analysis these NMDR MRSA strains were genotypically different. Genotypic tests, such as multiplex PCR for the mecA/nuc genes and DNA hybridization for the mecA gene, or phenotypic monoclonal antibody-based tests to detect penicillin-binding protein 2a (PBP2a) offer advantages for problematic isolates in detecting or confirming low-level phenotypic heterogeneous mecA expression of oxacillin and methicillin resistance in NMDR MRSA.
authors: Department of Infectious Diseases, Faculty of Medicine, University of Sydney, 2006 New South Wales, Australia. JMerlino@med.usyd.edu.au
authors: John
authors: J
authors: Merlino
authors: Jason
authors: J
authors: Watson
authors: Barbara
authors: B
authors: Rose
authors: Mary
authors: M
authors: Beard-Pegler
authors: Thomas
authors: T
authors: Gottlieb
authors: Ross
authors: R
authors: Bradbury
authors: Colin
authors: C
authors: Harbour
mesh_terms: D001426:Bacterial Proteins; D004269:DNA, Bacterial / Q000235:genetics / Q000302:isolation & purification; D018432:Drug Resistance, Multiple; D004720:Endonucleases / Q000235:genetics; D005838:Genotype; D016106:Methicillin Resistance / Q000502:physiology*; D008826:Microbial Sensitivity Tests; D008836:Micrococcal Nuclease; D009517:New South Wales; D010068:Oxacillin / Q000494:pharmacology*; D010403:Penicillin Resistance / Q000502:physiology*; D046915:Penicillin-Binding Proteins; D010406:Penicillins / Q000494:pharmacology*; D010641:Phenotype; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013211:Staphylococcus aureus / Q000187:drug effects*
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D046915:Penicillin-Binding Proteins; D010406:Penicillins; C487789:mecA protein, Staphylococcus aureus; C074002:nuc protein, staphylococcus; D004720:Endonucleases; D008836:Micrococcal Nuclease; D010068:Oxacillin