Lethal Microbial Synergism in Intra-abdominal Infections: Escherichia coli and Bacteroides fragilis

Ori D. Rotstein, Timothy L. Pruett, Richard L. Simmons

Research output: Contribution to journalArticlepeer-review

33 Scopus citations


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 (2×109 colony-forming units/clot) to E coli (2 ×108 colonyforming 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.

Original languageEnglish (US)
Pages (from-to)146-151
Number of pages6
JournalArchives of Surgery
Issue number2
StatePublished - Feb 1985


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