Rapid recognition of methicillin-resistant Staphylococcus aureus by use of automated test systems

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Abstract
The ability to rapidly recognize methicillin-resistant S. aureus by use of 2 automated instrument systems, the MS-2 system and the AutoMicrobic system, was evaluated on a collection of 95 methicillin-resistant S. aureus isolates recovered from patients in at least 6 geographical areas of the USA. Isolates were simultaneously tested with both systems, and the results were compared with minimum inhibitory concentration (MIC) tests performed by the National Committee for Clinical Laboratory Standards agar dilution method. Methicillin-resistant S. aureus isolates were defined as those with a methicillin MIC .gtoreq. 8 .mu.g/ml by the reference procedure. With the AutoMicrobic system, 94.7% of 95 methicillin-resistant S. aureus isolates were detected, and, with the MS-2 system, 91.6% of the isolates were detected. Isolates with methicillin MIC .gtoreq. 32 .mu.g/ml were readily detected with both systems (41 of 42 isolates). Of 53 isolates from 3 locales with methicillin MIC of 8 or 16 .mu.g/ml, 90.6% (48) were detected by the AutoMicrobic system, whereas 86.8% (46) were detected by the MS-2 system. A program update which has been added to the MS-2 system prints a warning message indicating possible methicillin-resistant S. aureus with isolates which demonstrate multiple antibiotic resistance (.gtoreq. 4 drugs other than methicillin). This warning message would have provided presumptive recognition of 6 of 8 isolates with discrepant results for methicillin by the MS-2 system.