Presence of the bla_Z beta-lactamase gene in isolates of Staphylococcus aureus that appear penicillin susceptible by conventional phenotypic methods

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.