Effect of Bacteroides fragilis on mortality induced by Escherichia coli in an experimental infection treated with cefotaxime, aztreonam or gentamicin.

The possibility that beta-lactamase-producing strains of Bacteroides fragilis can protect Escherichia coli from cefotaxime was studied in an in-vivo model of peritoneal infection in rats. The protective effect of cefotaxime, aztreonam and gentamicin in peritonitis induced by E. coli alone or combined with B. fragilis was evaluated by analysing mortality at 24 and 48 h after bacterial inoculation and treating the animals with two doses of each antibiotic. Comparisons, by drugs, at 24 and 48 h revealed that a statistically significant high mortality rate was obtained at 48 h when mixed infections were treated with cefotaxime, a drug very active in the infection caused by E. coli alone. Infections by mixed flora or E. coli alone treated with aztreonam or gentamicin did not show any significant difference in mortality rate analysed at 24 or 48 h. These in-vivo results confirm previous in-vitro studies and suggest that cefotaxime could be inactivated in mixed infections if a beta-lactamase-producing strain, such as B. fragilis, is involved in a clinical infection.