Effect of combinations of penicillin and aminoglycosides on Streptococcus faecalis: a comparative study of seven aminoglycoside antibiotics.

The 50% inhibitory concentrations (IC50) of benzylpenicillin, streptomycin, sisomicin, gentamicin, tobramycin, kanamycin, amikacin, and butirosin were determined for 58 clinical isolates of Streptococcus faecalis, 28 of which were recovered from cultures of blood samples from patients with endocarditis. The IC50 of streptomycin was less than 100 microng/ml for 42 strains, 192-10,000 microng/ml for eight, and larger than or equal to 10,000 micron/ml for eight. One isolate that was highly resistant to streptomycin was also highly resistant to kanamycin and butirosin. Extraordinarily high resistance to the other aminoglycosides was not observed. The bactericidal effects of combinations of penicillin and aminoglycosides were studied in 20 strains of S. faecalis that represented different levels of resistance to streptomycin. Significant enhancement of the effect of the combination of penicillin and streptomycin was found only in strains with an IC50 of smaller than or equal to 190 microng/ml. Combinations of penicillin and sisomicin, gentamicin, or tobramycin were effective even against strains that were highly resistant to streptomycin and kanamycin.