PHENOTYPIC DETECTION OF MEC A-POSITIVE STAPHYLOCOCCAL BLOOD-STREAM ISOLATES - HIGH-ACCURACY OF SIMPLE DISK DIFFUSION TESTS

Detection of oxacillin-resistance in staphylococci, by phenotypic meth ods remains problematic. Although standardized susceptibility test met hods are adequate for Staphylococcus aureus, many are less satisfactor y for the coagulase-negative staphylococci (CNS). We have studied 108 consecutive blood culture isolates of staphylococci. The mec A gene wa s detected by PCR in one S. aureus and 55 CNS isolates. Susceptibility testing was performed as follows: oxacillin (1-mu g), ceftizoxime (30 -mu g), and cephalothin (30-mu g) by disk diffusion; oxacillin, ceftiz oxime, cephalothin, methicillin, ampicillin, ampicillin/sulbactam, pen icillin, cefazolin, imipenem, and meropenem by the broth microdilution method. In addition, isolates were tested by the oxacillin agar scree n plate method. The single oxacillin-resistant S. aureus strain was de tected by all oxacillin susceptibility test methods and by the ceftizo xime disk and MIC methods. Two oxacillin-susceptible S. aureus were in termediate (minor error) by ceftizoxime broth microdilution (MIC, 16 m u g/mL). The most sensitive, simple phenotypic methods for detection o f oxacillin-resistant CNS (mec A positive) were as follows: oxacillin disk diffusion at 98%, oxacillin screen plate at 91%, oxacillin broth microdilution at 87%, ceftizoxime disk diffusion at 100%, ceftizoxime broth microdilution at 87%, and methicillin broth microdilution at 83% . These results indicate that oxacillin and ceftizoxime disk diffusion tests are the most accurate phenotypic methods in routine clinical us e for detection of oxacillin-resistant CNS. Oxacillin broth microdilut ion MIC testing (2% NaCl supplement) would perform more satisfactorily (100% sensitivity) with an adjusted interpretive breakpoint at less t han or equal to 0.5 mu g/mL, in contrast to the lower accuracy of the ''so-called'' reference agar screen test.