Bactericidal activity of antibiotics alone and in combination against Enterococcus faecalis in a pharmacodynamic...

We evaluated the bactericidal kinetics of teicoplanin, mezlocillin, netilmicin, and ciprofloxacin alone and in dual combinations against strains of Enterococcus faecalis susceptible or resistant to ampicillin in a pharmacodynamic model reproducing in bacterial culture in active human plasma or Mueller-Hinton broth the serum kinetics of these antibiotics in humans. Killing was not different in cultures grown in plasma and those grown in broth. Antibiotics used alone had no or low bactericidal activity except for high-dose ciprofloxacin (600 mg intravenously [iv] twice daily or 750 mg orally twice daily), which achieved a 3- to 4-log reduction in colony-forming units (cfu). Netilmicin was equally active at 6 mg/kg once a day or 2 mg/kg three times daily in all combinations. No major increase in bactericidal activity was detected in any combination that included mezlocillin. Maximal synergistic killing was observed for the combination of teicoplanin plus netilmicin (both at three-times daily and once-daily dosing), which sterilized the bacterial cultures (initial inoculum, 10(6) cfu/mL). Combinations of ciprofloxacin at 600 mg iv twice daily and 750 mg orally twice daily plus either teicoplanin or netilmicin were less synergistic but equally effective in total killing as a result of the high bactericidal activity of ciprofloxacin alone.