The protective effects of the probiotic Bifidobacterium lactis HN019 against Escherichia coli O157:H7 were investigated in murine challenge infection models. BALB/c or C57BL/6 mice were fed milk-based diets supplemented with B. lactis HN019 (3 x 10(8) cfu/g) for 7 days prior to and following oral challenge with E. coli O157:H7. Behavioral parameters (morbidity, feed intake) were measured for 7 days following challenge; immunological responses (phagocytosis, antibody) and pathogen translocation were measured in a sub-sample of ostensibly healthy animals 1 week post-challenge. Results showed that HN019-fed mice maintained significantly higher post-challenge feed intake and exhibited a lower cumulative morbidity rate, compared to control mice which did not receive the probiotic. Significantly higher proportions of phagocytically active cells in the blood and peritoneum, and higher intestinal tract IgA anti-E. coli antibody responses, were recorded among HN019-fed mice compared to controls. Among HN019-fed mice, pathogen translocation was identified in one of five BALB/c and one of five C57 mice; the comparative figures in control mice were two of five and three of five, respectively, and the mean bacterial burdens in these mice were over 100-fold higher than in HN019-fed mice. These results demonstrate that HN019 can reduce the severity of infection due to the enterohemolytic pathogen E. coli O157:H7, and suggest that this reduction may be associated with enhanced immune protection conferred by the probiotic.