Ampicillin and ampicillin-sulbactam dilution tests with mixed cultures of Bacteroides fragilis, Escherichia coli and Enterococcus.

Bacterial interactions in mixed infections may compromise antimicrobial therapy. The in vitro bactericidal activity of ampicillin and ampicillin-sulbactam against aerobic/anaerobic mixed cultures of Bacteroides fragilis, Escherichia coli and Enterococcus spp. was studied by means of broth dilution tests. The MBC of ampicillin for Enterococcus faecalis 6 was 0.25 mg/l when tested singly; in association with B. fragilis 1, however, the MBC for E. faecalis 6 was 16 mg/l. When tested singly E. coli 9 and B. fragilis 1 were both killed by ampicillin at a concentration of 4 mg/l in combination with 1 mg/l sulbactam. When both strains were associated, the MBC for B. fragilis 1 rose to > 256 mg/l. Results obtained indicate that the behaviour of bacteria as determined in pure cultures is not necessarily identical with the antibiotic susceptibility present in mixed cultures. It was shown that, besides beta-lactamase production by involved bacteria, other factors contribute to the alteration of bacterial susceptibility in mixed cultures. Synergistic and antagonistic effects between associated organisms were observed. For example, it was found that two moderately sensitive bacterial strains were resistant in mixed culture.