Bactericidal action of gentamicin against enterococci that are sensitive, or exhibit low- or high-level resistance to gentamicin.

Treatment of serious enterococcal infection involves the use of penicillin-aminoglycoside combination therapy if the aminoglycoside minimum inhibitory concentration (MIC) is < or = 2000 micrograms/ml, and the organism is susceptible to penicillin or ampicillin. We evaluated killing of 15 enterococci that differ in their susceptibility to gentamicin using time-kill studies at different gentamicin concentrations. Sensitive strains had a uniform population killed by gentamicin concentrations equal to or above the MIC. Low-level resistant strains (MIC > or = 8 but < or = 2000 micrograms/ml of gentamicin) had a diverse population with large numbers of cells killed at one-half the MIC, while the highly resistant strains (MIC > 2000 micrograms/ml) showed no killing by any concentration of gentamicin.