Antibiotic Synergism of Enterococci

Not all enterococci show synergism between penicillin and streptomycin or kanamycin, but most patients receive combined therapy empirically since tests for synergism are difficult to perform. Enterococci showing synergism are considered "resistant" to streptomycin and kanamycin, but usually have not been tested with concentrations higher than 100μ/ml. We had found that strains not showing synergism are actually highly resistant to these antibiotics, in contrast to strains that do show synergism. Of 30 strains where synergism occurred between penicillin G potassium and streptomycin sulfate when killing curves were performed, 27 were inhibited by 250μg/ml. Seventeen strains not showing synergism required 6,000μg/ml or more for inhibition and 11 grew in 50,000μg/ml Forty-four strains showing synergism between penicillin and kanamycin sulfate were inhibited by 250μg/ml or less of kanamycin sulfate, and 5 strains not showing synergism had minimal inhibitory concentrations of 25,000μg/ml or higher.