BACTERICIDAL ACTION OF GENTAMICIN AGAINST ENTEROCOCCI THAT ARE SENSITIVE, OR EXHIBIT LOW-LEVEL OR HIGH-LEVEL RESISTANCE TO GENTAMICIN

Treatment of serious enterococcal infection involves the use of penici llin-aminoglycoside combination therapy if the aminoglycoside minimum inhibitory concentration (MIC) is less than or equal to 2000 mu g/ml, and the organism is susceptible to penicillin or ampicillin. We evalua ted killing of 15 enterococci that differ in their susceptibility to g entamicin using time-kill studies at different gentamicin concentratio ns. Sensitive strains had a uniform population killed by gentamicin co ncentrations equal to or above the MIC. Low-level resistant strains (M IC greater than or equal to 8 but less than or equal to 2000 mu g/ml o f gentamicin) had a diverse population with large numbers of cells kil led at one-half the MIC, while the highly resistant strains (MIC >2000 mu g/ml) showed no killing by any concentration of gentamicin.