Lethal microbial synergism in intra-abdominal infections. Escherichia coli and Bacteroides fragilis.

The ability of Bacteroides fragilis and Escherichia coli to produce synergistic mortality when mixed into intraperitoneal (IP) fibrin clots was tested in rats. The addition of B fragilis (2 X 10(9) colony-forming units/clot) to E coli (2 X 10(8) colony-forming units/clot) in the clot significantly enhanced both early and late mortality rates when compared to either E coli or B fragilis alone. Multiple washings of B fragilis prior to mixing with E coli in the clot delayed the enhancement of lethality from 24 to 48 hours. By seven days, washed B fragilis was as synergistic with E coli as unwashed B fragilis plus E coli. Furthermore, unwashed killed B fragilis was as synergistic when mixed with E coli in the fibrin clot as unwashed living B fragilis. However, washed dead B fragilis plus E coli produced no greater mortality than E coli alone. The lethality of an IP clot containing E coli was significantly increased when B fragilis was mixed with it in the same clot, injected free IP, and or implanted into a separate IP clot. Intraperitoneal E coli-fibrin clot lethality was not increased by subcutaneous B fragilis and was only slightly enhanced by intravenous B fragilis inoculation. The strain of B fragilis used in these studies did not produce fibrinolysins at any concentration. The data support the idea that synergistic mortality between E coli and B fragilis in this model is caused by a heat-stable surface factor produced by B fragilis, which acts to increase the lethal effects of E coli.