Comparative bactericidal activity of penicillin-netilmicin and penicillin-gentamicin against enterococci.

The synergistic bactericidal activity of netilmicin in combination with penicillins was compared with that of gentamicin and streptomycin against 16 clinical isolates of enterococci. In a macrodilution chessboard assay against multiple combinations of penicillin, gentamicin and netilmicin, bactericidal activity uniformly occurred at two- to four-fold lower concentrations of penicillin + netilmicin than with penicillin + gentamicin. With four strains, penicillin + netilmicin produced up to ten-fold greater killing than penicillin + gentamicin after 6 h of incubation. With one strain, 3 X log10 greater killing occurred with netilmicin after 24 h. Netilmicin was two- to four-fold more active than gentamicin when combined with penicillinase-resistant penicillins. The bactericidal activity of netilmicin against enterococci is equal to or greater than that of gentamicin, either alone or when combined with penicillins.