Bifidobacterium strains from resident infant human gastrointestinal microflora exert antimicrobial activity

Background and aims-The gastrointestinal microflora exerts a barrier effect against enteropathogens. The aim of this study was to examine if bifidobac teria, a major species of the human colonic microflora, participates in the barrier effect by developing antimicrobial activity against enterovirulent bacteria. Methods-Antibacterial activity was examined in vitro against a wide range o f Gram negative and Gram positive pathogens. Inhibition of Salmonella typhi murium SL1334 cell association and cell invasion was investigated in vitro using Caco-2 cells. Colonisation of the gastrointestinal tract in vivo by b ifidobacteria was examined in axenic C3/He/Oujco mice. Antimicrobial activi ty was examined in vivo in axenic C3/He/Oujco mice infected by the lethal S typhimurium C5 strain. Results-Fourteen human bifidobacterium strains isolated from infant stools were examined for antimicrobial activity. Two strains (CA1 and F9) expresse d antagonistic activity against pathogens in vitro, inhibited cell entry, a nd killed intracellular S typhimurium SL1344 in Caco-2 cells. An antibacter ial component(s) produced by CAI and F9 was found to be a lipophilic molecu le(s) with a molecular weight of less than 3500. In the axenic C3/He/Oujco mice, CAI and F9 strains colonised the intestinal tract and protected mice against S typhimurium C5 lethal infection. Conclusion-Several bifidobacterium strains from resident infant human gastr ointestinal microflora exert antimicrobial activity, suggesting that they c ould participate in the "barrier effect" produced by the indigenous microfl ora.