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Background: Penicillin resistance in S. aureus is mediated by four types of Penicillinases encoded by the blaZ gene. Because penicillin is considered superior to oxacillin in those rare isolates with true penicillin susceptibility, correct determination of penicillinase is important. Laboratories using the Vitek 2 for susceptibility testing of S. aureus have to use an additional test for beta-lactamase detection for those isolates with an MIC of <= 0.12 mg/l.
Methods: In our ongoing study we investigated several methods for penicillinase detection, like size and character of penicillin inhibition zones, nitrocefin testing, cloverleaf assays, starch iodine plates and an in-house blaZ-PCR for isolates with an MIC <= 0.12 mg/l in the Vitek 2.
An isolate was considered penicillinase positive if either the blaZ-PCR, the nitrocefin test read after 60 min, the cloverleaf assay or the penicillin zone edge appearance suggested the presence of a penicillinase.
Results: So far 175 isolates have been tested. Sensitivities were 85.3% for penicillin zone edge appearance, 79% for an penicillin inhibition zone diameter <= 28 mm, 58.8% for the nitrocefin test read after 60 min, 85.3% for the cloverleaf assay, 67.6% for starch iodine plates and 91% for the blaZ-PCR. Isolates with a MIC of 0.12 mg/l and 0.06 mg/l in the Vitek 2, respectively were penicillinase positive in 26% and 6.9%.
Conclusion: Cloverleaf assays, penicillin zone edge determination and blaZ-PCR are superior to starch iodine plate and nitrocefin testing for the detection of penicillinase in S. aureus.
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