Combined effect of benzylpenicillin and gentamicin/netilmicin on Streptococcus faecalis.

The 50% inhibitory concentrations (IC50) of gentamicin and netilmicin were determined for 55 clinical isolates of Streptococcus faecalis, approximately half of which were recovered by culture of blood samples from patients with endocarditis. The IC50 for gentamicin was less than 5 micrograms/ml for 22 strains and less than 40 micrograms/ml for all strains, and for netilmicin the IC50 was less than 5 micrograms/ml for 51 strains and less than 40 micrograms/ml for all strains. Eight out of the 55 strains were found to have IC50 values greater than 10(4) micrograms/ml for streptomycin (3). The bacterial effect of combinations of benzylpenicillin and gentamicin or netilmicin was studied in 19 strains of S. faecalis which represented different levels of resistance to streptomycin. Netilmicin was as effective as gentamicin in combination with benzylpenicillin against all strains.