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 ofBacteroides fragilis withEscherichia coli orEnterococcus faecalis revealed MBCs of 64 mg/l for enoxacin. However, investigating metronidazole, MBCs forB. fragilis increased from 0.5–1 mg/l to 2–4 mg/l in association withE. coli. The combination of enoxacin with metronidazole restored MBCs of 0.5 and 1 mg/l for theB. fragilis strains.B. fragilis associated withE. faecalis showed MBCs of 2 to 64 mg/l for metronidazole, an increase of up to 64-fold. The enoxacin MBCs forE. coli in association withB. fragilis were up to eight-fold higher than forE. coli alone (2 mg/l compared to 0.25 mg/l). Association ofE. coli withE. faecalis andB. fragilis showed a low to moderate increase for MBCs ofE. coli when enoxacin was tested alone or in combination with metronidazole or clindamycin. In contrast, MBCs forE. faecalis did not change significantly with any of the associations or combinations tested. It is evident thatin vitro antimicrobial susceptibility of associations of pathogens can be modified by interactions between strains, and/or antimicrobials.