Characterization and comparison of two penicillinase-producing strains of Streptococcus (Enterococcus) faecalis

We identified two β-lactamase-positive enterococci. One strain was high-level (MIC, >2,000 μg/ml) gentamicin resistant; the other ws not (MIC, 12.5 μg/ml). β-Lactamase production was extrachromosomally mediated in both strains, and both strains showed an inoculum effect reversed by β-lactamase inhibitors. The strain lacking high-level gentamicin resistance showed synergistic killing with a combination of penicillin, clavulanic acid, and gentamicin.