Vancomycin resistance in Enterococcus gallinarum

The vancomycin resistance expressed by several strains of Enterococcus gallinarum was studied. Resistance was expressed constitutively, as demonstrated by analysis of growth and inhibition of peptidoglycan synthesis. E. gallinarum strains were moderately resistant to vancomycin (MIC, 16 micrograms/ml) but were as susceptible as vancomycin-susceptible enterococci to the glycopeptides, teicoplanin, A35512B, A47934, A4103A, and A41030E and the glycopeptide actaplanins A1, B2, and C1. Vancomycin resistance in E. gallinarum was inhibited by beta-lactam antibiotics at concentrations that saturated penicillin-binding protein 6 (PBP 6), as demonstrated by binding competition experiments. Spontaneous mutants (frequency, 10(-8)) were two- to fourfold more resistant to beta-lactam inhibition of vancomycin resistance than the parent strain. PBP binding competition experiments suggested that PBP 6 in the mutants bound less cefotaxime, while binding of penicillin and cefoxitin was unaffected. Both a bioassay method and high-performance liquid chromatography showed that E. gallinarum membranes have enzymatic activity which modifies a model pentapeptide yielding a product that is thought to be a tetrapeptide. This activity could be a D,D-carboxypeptidase. In both the parent E. gallinarum strain and its derivatives that were resistant to the synergistic drug combination, the activity was inhibited by beta-lactams at concentrations which correlated with those that inhibit vancomycin resistance and those that saturate PBP 6. These results suggest the possibility that PBP 6 may be involved in the vancomycin resistance of E. gallinarum and that the putative D,D-carboxypeptidase activity seen in E. gallinarum membranes may be attributable to PBP 6.