In-vitro study of the synergy between beta-lactam antibiotics and glycopeptides against enterococci

The synergy between glycopeptides and beta-lactams was studied using different techniques such as broth macrodilution, killing curves and agar dilution combined with agar diffusion. Two glycopeptide-resistant enterococci isolated from different clinical samples were used. Results showed different effects with significant changes in MICs. Antibacterial activity was related to the concentration of glycopeptide and beta-lactam for Enterococcus faecalis 8253, while for Enterococcus faecium 8072 a paradoxical effect was observed. With this strain, the best synergic effect was detected at teicoplanin concentrations of 1-4 mg/L, but antibacterial activity was reduced at concentrations of 8, 16 and 32 mg/L. No synergic effect was observed with vancomycin. The combination of agar dilution with agar diffusion techniques may constitute a simple method for routine detection of synergic effects between glycopeptides and beta-lactams.