Beta-lactamase production may not be reliably detected by commonly used susceptibility testing methods such as Kirby-Bauer penicillin disk diffusion and nitrocefin beta-lactamase detection. We assayed 105 apparently penicillin-susceptible Staphylococcus aureus isolates using multiple methods to detect beta-lactamase production. The bla(Z) beta-lactamase gene was detected by polymerase chain reaction in 10 (9.5%) of the 105 isolates. The average disk diffusion zone diameter was 34 and 38 mm for the bla(Z)-positive and -negative isolates, respectively (P < 0.001). Qualitative description of the zone edge was observer-dependent. The "cloverleaf assay" was positive in 6 of the 10 phenotypically susceptible isolates possessing bla(Z). The results of this study suggest that conventional methods for S. aureus penicillin susceptibility testing may not reliably detect penicillin resistance in all isolates; however, increasing the disk diffusion zone size interpretive criteria to 35 mm for this antimicrobial/organism combination from the current 29-mm breakpoint may improve the sensitivity of phenotypic penicillin susceptibility testing.
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