Investigation of Oxacillin-Hydrolyzing Beta-Lactamase in Borderline Methicillin-Resistant Clinical Isolates of <i>Staphylococcus aureus</i>

Abstract

Background: Mechanisms of borderline resistance of Staphylococcus aureus to penicillinase-resistant penicillins (PRPs) may include hyperproduction of classical penicillinase and/or production of β-lactamase hydrolyzing also PRPs. Methods: β-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 µg/ml, which were reduced by sulbactam and/or clavulanic acid in the case of four isolates producing large quantities of inducible, type A β-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.

Prevalence of Mastitis and Phenotypic Characterization of Methicillin-Resistant Staphylococcus aureus in Lactating Dairy Cows of Selected Dairy Farms in and Around Adama Town, Central Ethiopia

“… 51 This might be due to the fact that resistance to Methicillin in these isolates was not only due to the mec A gene since other modes of resistance to Methicillin such as the presence or over expression of ß-lactamase enzymes and chromosomal mutations have been documented. 52 , 53 The differences in the antimicrobial resistance rates to the various drugs observed might reflect a difference in regional antimicrobial usage and subsequent epidemiology due to inappropriate use of antibiotics in some regions. The choice of antibiotic for therapy of MRSA infections is usually complicated.…”

Staphylococci isolated from animals and food with phenotypically reduced susceptibility to β-lactamase-resistant β-lactam antibiotics

“…The so-called borderline-resistant strains that produce large amounts of β-lactamase and may hydrolyse methicillin, oxacillin and sometimes β-lactamase inhibitors as well appear as low-level methicillin resistant strains (McDougal and Thornsberry, 1986). However, it was shown that besides β-lactamase hyperproduction, a small increase in intrinsic resistance is required for the emergence of borderline resistance (Barg et al, 1991;Gál et al, 2001). Among clinical BORSA isolates methicillinase-producing strains were detected (Massida et al, 1991).…”

Novel mutations in penicillin-binding protein genes in clinical Staphylococcus aureus isolates that are methicillin resistant on susceptibility testing, but lack the mec gene

“…4 S. aureus isolates with other types of resistance to β-lactams have been described and are known as borderline oxacillin-resistant S. aureus (BORSA) or intrinsically resistant S. aureus . 5,6 Resistance in some BORSA isolates is attributed to the presence or overexpression of β-lactamase enzymes, 6,7 while in intrinsically resistant and some BORSA isolates it is mediated by chromosomal mutations. 5,8,9 Furthermore, in vitro studies generating β-lactam-resistant isolates under the selection of β-lactam antibiotics have also identified a number of other genes able to mediate intrinsic β-lactam resistance.…”