Ampicillin and ampicillin-sulbactam dilution tests with mixed cultures ofBacteroides fragilis, Escherichia coli andEnterococcus

Bacterial interactions in mixed infections may compromise antimicrobial therapy. Thein vitro bactericidal activity of ampicillin and ampicillin-sulbactam against aerobic/anaerobic mixed cultures ofBacteroides fragilis, Escherichia coli andEnterococcus spp. was studied by means of broth dilution tests. The MBC of ampicillin forEnterococcus faecalis 6 was 0.25 mg/l when tested singly; in association withB. fragilis 1, however, the MBC forE. faecalis 6 was 16 mg/l. When tested singlyE. coli 9 andB. 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 forB. 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 β-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.