Reciprocal synergy between Escherichia coli and Bacteroides fragilis in an intra-abdominal infection model

The synergic relationship between Escherichia coli and Bacteroides fragilis was examined in a model of intra-abdominal abscess formation. The addition of B. fragilis to E. coli in the fibrin clot inoculum increased abscess weight and residual numbers of E. coli in the abscess at 7 days. In a reciprocal fashion, E. coli was capable of enhancing B. fragilis persistence in abscesses. Neither heat-killed E. coli nor heat-killed B. fragilis was able to mimic the synergic effect of its live counterpart. Furthermore, B. fragilis culture filtrate was unable to reproduce the ability of live B. fragilis to act synergically with E. coli. For B. fragilis to act synergically with E. coli, it had to be inoculated locally with E. coli in the peritoneal cavity, indicating that an effect on systemic resistance by B. fragilis was an unlikely mechanism for the production of bacterial synergy. These studies suggest that the synergic relationship between bacteria in polymicrobial infections is a complex one, resulting from intimate interactions between bacteria and the host in the local milieu of the infection.