The spectrum of Escherichia coli--Bacteroides fragilis pathogenic synergy in an intraabdominal infection model.

Pathogenic synergy between Escherichia coli and Bacteroides fragilis was investigated in an intraabdominal infection model. Defined inocula of E. coli and B. fragilis, alone or in combination, were enmeshed within a fibrin clot and surgically implanted into the peritoneal cavity of rats. A spectrum of bacterial synergy ranging from synergistic abscess formation to synergistic lethality was demonstrated using this model. The type of synergy exhibited was dependent upon the initial E. coli inoculum. When combined with B. fragilis, high inocula of E. coli (greater than 10(8) cfu/clot) produced synergistic lethality while low inocula (2 x 10(2) to 2 x 10(7) cfu/clot) resulted in synergistic abscess formation. With respect to abscess formation, there was reciprocal synergy between E. coli and B. fragilis. Abscesses resulting from mixed inocula were larger and had significantly higher numbers of E. coli and B. fragilis than abscesses initiated by monomicrobial inocula. These studies define a clinically relevant model of bacterial interactions in the setting of intraabdominal infection and suggest that conclusions drawn from experimental models of bacterial synergy should consider the type of model examined, the strains of bacteria studied, and the number of bacteria inoculated.