Comparative killing activity and postantibiotic effect of streptomycin combined with ampicillin, ciprofloxacin, imipenem, piperacillin or vancomycin against strains of Streptococcus faecalis and Streptococcus faecium.

Nine strains of Streptococcus faecalis and Streptococcus faecium were studied with respect to ampicillin, ciprofloxacin, imipenem, piperacillin, vancomycin and streptomycin. Two strains were highly resistant (MIC greater than or equal to 2,000 micrograms/ml) to streptomycin. Evaluation and comparison of the killing activity with killing curves, and duration of the postantibiotic effect (PAE) after exposure for 1 h with regrowth curves was done with combinations of antibiotics or alone. The overall killing effect of ciprofloxacin with streptomycin was antagonistic, whereas synergism (greater than or equal to one log10 decrease in viable counts) was observed in streptomycin-susceptible strains with combinations of streptomycin and ampicillin, imipenem, piperacillin or vancomycin. In addition, prolongation of PAE (greater than or equal to 0.5 h) was seen only in streptomycin-susceptible strains. Thus, seven (100%) strains showed a synergistic increase in PAE to combinations with ampicillin and vancomycin, three (43%) to imipenem, four (57%) to piperacillin, but none to the combination of streptomycin and ciprofloxacin. A significant correlation was observed between the magnitude of increased killing and the extent of increase in recovery period with combinations of streptomycin with either ampicillin or vancomycin.