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Comparison of methods for penicillinase detection in S. aureus

Abstract number: 1733_1134

Kaase M., Lenga S., Szabados F., Sakinc T., Anders A., Friedrich S., Gatermann S.G.

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 b-lactamase detection for those isolates with an MIC of ≤ 0.12 mg/L.

Methods: We investigated several methods for penicillinase detection, like size and character of penicillin inhibition zones, nitrocefin testing, cloverleaf assay, 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 15 min, the cloverleaf assay or the penicillin zone edge appearance suggested the presence of a penicillinase.

Results: Of 274 isolates 50 (18.2%) were penicillinase positive. Sensitivities were 80% for penicillin zone edge appearance, 62% for an penicillin inhibition zone diameter ≤28 mm, 52% for the nitrocefin test read after 15 min, 74% for the cloverleaf assay, 58% for starch iodine plates and 88% 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 24.2% and 6.2%.

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.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Subject:
Location: ICC, Munich, Germany
Presentation type:
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