Comparison of the action of ampicillin and benzylpenicillin on enterococci in vitro.

The bactericidal activity of benzylpenicillin and ampicillin on 21 strains of enterococci was evaluated and compared to the activity of these drugs in combination with streptomycin (20 mug/ml). On a weight basis, ampicillin was about twice as effective as benzylpenicillin. Neither of the drugs was rapidly and completely bactericidal for any of the 21 strains of enterococci when used alone. The addition of streptomycin greatly enhanced the early bactericidal rate achieved with any given amount of either penicillin and permitted the elimination of viable organisms in vitro. These results suggest that, for the time being, combined antibiotic therapy might be desirable in enterococcus endocarditis and that ampicillin, although more effective than benzylpenicillin, should not be relied upon as a single drug in that disease.