BACKGROUND Escherichia coli is part of the normal gastrointestinal microflora which exerts a barrier effect against enteropathogens. SeveralE coli strains develop a protective effect against other Enterobacteriaceae.

AIMS TwoE coli strains, EM0, a human faecal strain, and JM105 K-12 were tested for their ability to prevent in vivo and in vitro infection by Salmonella typhimurium C5.

RESULTS EMO expresses haemolysin and cytotoxic necrotising factor in vitro. In vitro, the two strains did not prevent the growth of C5 by secreted microcins or modified cell invasion of C5. In vivo, establishment of EM0 or JM105 in the gut of germfree mice resulted in a significant increase in the number of surviving mice: 11/12 and 9/12, respectively, at 58 days after infection (2×10⁶/mouse) versus 0/12 in control germfree group at 13 days after infection. Colonisation level and translocation rate of C5 were significantly reduced during the three days after infection. In contrast, no reduction in faecal C5 excretion was observed in C5 infected conventional mice (1×10⁸/mouse) receiving the EM0 or JM105 cultures daily.

CONCLUSIONS Establishment of E coli strains, which do not display antimicrobial activity, protects germfree mice against infection and delays the establishment of C5 in the gut. Possible mechanisms of defence are discussed.