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Aims: The antagonistic activity of the Escherichia coli strain H22 against enteric bacteria was studied both in vitro and in vivo. Methods and Results: In vitro, bacterial strains belonging to seven of nine genera of the family Enterobacteriaceae ( Enterobacter, Escherichia, Klebsiella, Morganella, Salmonella, Shigella and Yersinia) were inhibited by the strain H22. Six days after simultaneous oral inoculation in germ-free mice, E. coli strain H22 reduced the faecal population of Shigella flexneri 4 to undetectable levels ( P < 0·05). In ex vivo assay, inhibitory zones against Sh. flexneri 4 were observed around faecal samples from mice inoculated with E. coli strain H22. The in vitro inhibition of Sh. flexneri 4 was shown to be mediated by microcin C7. In addition to microcin C7, strain H22 was shown to produce aerobactin, new variants of colicins E1 and Ib, and bacteriophage particles with morphology similar to the phages of the family Myoviridae. Conclusions: Altogether, the properties of E. coli H22, observed both under in vitro and in vivo conditions, suggest its potential use as a probiotic strain for livestock and humans. Significance and Impact of the Study: The strain H22 was shown to produce several antimicrobial compounds with inhibitory capabilities against pathogenic or potentially pathogenic enterobacteria. [ABSTRACT FROM AUTHOR]