Association experiments with aerobic and anaerobic pathogens: a model of in-vitro susceptibility testing in mixed infections. Activity of enoxacin, clindamycin, and metronidazole.

In infections of polymicrobial etiology, it seems mandatory to combine an antibiotic with marginal activity against anaerobes with an anti-anaerobic drug, e.g. metronidazole or clindamycin. We investigated the effect of associations of anaerobic and facultatively anaerobic pathogens on MBCs of enoxacin, clindamycin, metronidazole, and combinations of enoxacin plus clindamycin or metronidazole. Single testing, and associations of Bacteroides fragilis with Escherichia coli or Enterococcus faecalis revealed MBCs of 64 mg/l for enoxacin. However, investigating metronidazole, MBCs for B. fragilis increased from 0.5-1 mg/l to 2-4 mg/l in association with E. coli. The combination of enoxacin with metronidazole restored MBCs of 0.5 and 1 mg/l for the B. fragilis strains. B. fragilis associated with E. faecalis showed MBCs of 2 to 64 mg/l for metronidazole, an increase of up to 64-fold. The enoxacin MBCs for E. coli in association with B. fragilis were up to eight-fold higher than for E. coli alone (2 mg/l compared to 0.25 mg/l). Association of E. coli with E. faecalis and B. fragilis showed a low to moderate increase for MBCs of E. coli when enoxacin was tested alone or in combination with metronidazole or clindamycin. In contrast, MBCs for E. faecalis did not change significantly with any of the associations or combinations tested. It is evident that in vitro antimicrobial susceptibility of associations of pathogens can be modified by interactions between strains, and/or antimicrobials.