Detection of methicillin-resistant Staphylococcus aureus by microdilution and disk elution susceptibility systems

To determine whether methicillin-resistant (MR) Staphylococcus aureus from different geographic areas are detected reliably by various commercially available microdilution broth and disk elution systems, 73 such isolates obtained from hospitals in 13 cities were tested by a reference method (agar dilution) and by the Microscan, API 3600S, Autobac I, and MS-2 systems. Both Eugonic broth and Low Thymidine Eugonic broth were used in the evaluation of the Autobac I, and two versions of the MS-2 were used. The proportions of isolates categorized as MR by the various methods were: agar dilution method, 99%; Microscan, 100% (if the suggested cut-off of the manufacturer was used); API 3600S, 96%; Autobac I, 84 to 93%; and MS-2, 54 to 68%. With the MS-2 system, isolates from Jackson, Miss., were classified as susceptible to methicillin more often than were strains from other cities. With the Autobac I (Eugonic broth), only 55% of isolates from Houston, Tex., were classified as MR, whereas 89% of isolates from all other cities were correctly classified as MR. With the API 3600S, strains from some cities were categorized as nafcillin susceptible, whereas strains from other cities were classified as resistant to nafcillin. The results of this study suggest that future evaluations of antimicrobial susceptibility testing systems should include MR strains of S. aureus from several geographic areas.