Efficacy of sulbactam in an in vitro model of mixed aerobic/anaerobic infections.

Bacterial interactions in mixed infections may interfere with antimicrobial therapy. Thein-vitro efficacy of ampicillin alone, combination sulbactam/ampicillin, and metronidazole was studied. Strains ofBacteroides fragilis, Escherichia coli, andEnterococcus faecalis, alone and in association, were tested by means of a broth dilution method. Minimal bactericidal concentrations (MBC) of ampicillin forB. fragilis 74 in association withE. coli 68 were up to 16-fold higher than forB. fragilis 74 alone (256 compared to 16 mg/l), but only 2-fold higher for sulbacta(5 mg/l)/ampicillin (0.5 and 0.25 g/l). Association ofB. fragilis 45 andE. faecalis 186 increased ampicillin MBC ofE. faecalis 186 from 2 to 16 mg/l, but the combination sulbactam/ampicillin restored activity of ampicillin. In association withE. faecalis, metronidazole MBCs ofB. fragilis increased up to 64-fold. Strains ofE. faecalis andE. coli were able to destroy 10 mg/l metronidazole within 8 to 20 h. The present experiments demonstrated effectiveness of sulbactam/ampicillin to inhibit β-lactamases of associated pathogens. Destruction of metronidazole byE. faecalis lends additional support to the use of the combination in aerobic/anaerobic infections includingE. faecalis.