Effect of prophylactic antibiotics upon mixed infections with Bacteroides fragilis.

Antimicrobial agents were used alone or in combinations to explore the effect of prophylactic antimicrobial therapy. Subcutaneous abscesses in mice were induced by single and mixed infections of Bacteroides fragilis, Staphylococcus aureus, Group A streptococci and Escherichia coli. The infected mice were treated with three doses of gentamicin, cefoxitin, metronidazole or clindamycin alone or else metronidazole or clindamycin in combination with gentamicin. Mice were sacrificed five days after inoculation and the bacterial contents of the abscesses were determined. Infection induced by a single bacteria always responded to appropriate antimicrobial therapy. However, in infections caused by two organisms, therapy directed at either the Bacteroides fragilis (with metronidazole or clindamycin) or Escherichia coli (with gentamicin) was effective in not only significantly reducing the colony forming units (CFU) of the target organism but also reducing the number of untreated bacteria. Clindamycin alone was effective in reducing the CFU of both components of mixed infections of Bacteroides fragilis with either Staphylococcus aureus or Group A streptococci. Cefoxitin alone and the combination of either clindamycin or metronidazole with gentamicin were effective against all mixed infections. These data support the need to provide coverage for all components of mixed infections with single or combination therapy.