Effects of Escherichia coli and Bacteroides fragilis on peritoneal host defenses

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Abstract
E. coli and B. fragilis are common copathogens in clinical intra-abdominal sepsis, yet it is unclear how they interact synergistically in vivo. It was determined whether E. coli and B. fragilis, in combination but not alone, could exert a detrimental effect on the peritoneal host defenses of translymphatic absorption and bacterial phagocytosis. These data indicated that nonviable E. coli (O18ab:K56 K7:- and O111:B4), Klebsiella pneumoniae, B. fragilis and B. thetaiotaomicron were handled in a similar fashion by both host defenses of the peritoneal cavity. The use of 2 x 108 nonviable radiolabeled E. coli as a tracer and either 2 x 108 B. fragilis or 2 x 109 E. coli(either viable or nonviable) as a competing agent to inhibit host defenses demonstrated that although clearance and phagocytosis could be inhibited, the inhibition occurred to a similar degree with either E. coli or B. fragilis. Thus, B. fragilis did not compete to any greater extent than E. coli did not for peritoneal clearance or opsonization and phagocytosis in vivo. These data indicate that bacterial synergy probably does not occur on the basis reduced peritoneal clearance or by a reduction in the opsonization and phagocytosis of either organism by the copathogen. These results provide indirect support for the hypothesis that in bacteria synergy, 1 organism directly stimulates the growth of the other, perhaps by providing a growth factor.