Challenge studies were set up feeding Lactobacillus casei and Lactobacillus acidophilus fermented milk and two different pathogenic microorganisms: Listeria monocytogenes and enteroinvasive Escherichia coli. Mice were fed for 8 consecutive days with fermented milk and then challenged with the pathogens. The survival rate in control mice was 62% for Listeria and 83% for E. coli, while 100% protection was observed for the 20 d per vial in treated mice. Colonization of the liver and spleen by E. coli was markedly inhibited by pretreatment with fermented milk; the pathogen was not detected on the 5th day postchallenge. In the Listeria challenged mice, the pathogen was present in 1 to 2 log units lower than control up the 10th day. The levels of antipathogen sera and intestinal antibodies were 2 to 4 times higher in the treated mice, with lower values in the Listeria treated mice. The mechanism of protection in both types of infections was discussed. The results obtained suggested that milk fermented with L. casei and L. acidophilus could be used as a prophylactic against selected infections.

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