Detection and expression of methicillin/oxacillin resistance in multidrug-resistant and non-multidrug-resistant Staphylococcus aureus in Central Sydney, Australia.
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.
Merlino
John
J
Department of Infectious Diseases, Faculty of Medicine, University of Sydney, 2006 New South Wales, Australia. JMerlino@med.usyd.edu.au
Watson
Jason
J
Rose
Barbara
B
Beard-Pegler
Mary
M
Gottlieb
Thomas
T
Bradbury
Ross
R
Harbour
Colin
C
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*
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