Investigation of oxacillin-hydrolyzing beta-lactamase in borderline methicillin-resistant clinical isolates of Staphylococcus aureus

Background: Mechanisms of borderline resistance of Staphylococcus aureus to penicillinase-resistant penicillins (PRPs) may include hyperproduction of classical penicillinase and/or production of beta -lactamase hydrolyzing also PRPs, Methods: beta -Lactamase activity of whole cells and purified enzymes was estimated spectrophotometrically and in isolated cytoplasmic membranes by bioassay with Bacillus subtilis as test strain. Results: Out of 53 clinical isolates of S. aureus, 18 showed oxacillin MIC values from 0.5 to 2 mug/ml, which were reduced by sulbactam and/or clavulanic acid in the case of four isolates producing large quantities of inducible, type A beta -lactamase. Cytoplasmic membranes isolated from these strains showed oxacillin-hydrolyzing activity. One of these strains was grown also in the presence of globomycin, an antibiotic known to interfere with the anchorage of membrane lipoproteins; this treatment eliminated the oxacillin-hydrolyzing activity. Conclusions: The resistance in these strains was due to a membrane-bound lipoprotein with oxacillin-hydrolyzing activity. Copyright (C) 2001 S.Karger AG,Basel.