Escherichia coli strains colonising the gastrointestinal tract protect germfree mice against Salmonella typhimurium infection

Background-Escherichia coli is part of the normal gastrointestinal microflo ra which exerts a barrier effect against enteropathogens. Several E coli st rains develop a protective effect against other Enterobacteriaceae. Aims-Two E coli strains, EMO, a human faecal strain, and JM105 K-12 were te sted for their ability to prevent in vivo and in vitro infection by Salmone lla typhimurium C5. Methods-Inhibition of C5 cell invasion by E coli was investigated in vitro using Caco-2/TC7 cells. The protective effect of E coli was examined in viv o in germfree or conventional C3H/He/Oujco mice orally infected by the leth al strain 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 mic rocins or modified cell invasion of C5. In vivo, establishment of EMO or JM 105 in the gut of germfree mice resulted in a significant increase in the n umber of surviving mice: 11/12 and 9/12, respectively, at 58 days after inf ection (2x10(6)/mouse) versus 0/12 in control germfree group at 13 days aft er infection. Colonisation level and translocation rate of C5 were signific antly reduced during the three days after infection. In contrast, no reduct ion in faecal C5 excretion was observed in C5 infected conventional mice (1 x10(8)/mouse) receiving the EMO or JM105 cultures daily. Conclusions-Establishment of E coli strains, which do not display antimicro bial activity, protects germfree mice against infection and delays the esta blishment of C5 in the gut. Possible mechanisms of defence are discussed.