Protective Effect ofLactobacillus caseiStrain Shirota on Shiga Toxin-ProducingEscherichia coliO157:H7 Infection in Infant Rabbits

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Abstract
We examined colonization patterns of Shiga toxin-producingEscherichia coli(STEC), concentrations of Shiga toxins (Stxs) and specific immunoglobulin A (lgA) against Stxs and STEC bacterial cell surface antigen in various portions of the gastrointestinal tract in an infant rabbit infection model. After inoculation of 3-day-old infant rabbits with STEC strain 89020087 at low doses (∼103CFU/body), numbers of colonizing STEC bacteria and concentrations of Stxs in the intestine increased dramatically and the animals developed diarrhea within a couple of days after infection. Daily administration ofLactobacillus caseifrom the day of birth dramatically decreased the severity of diarrhea and lowered STEC colonization levels in the gastrointestinal tract 100-fold day 7 after infection. Both Stx1 and Stx2 concentrations in the intestines and histological damage to the intestinal mucus induced by STEC infection were decreased by the administration ofL. casei. Examination of the concentrations of volatile fatty acids and pH of the intestinal contents revealed that the protective effect ofL. caseiadministration against STEC infection was not due to fermented products such as lactic acid in the gastrointestinal tract. Administration ofL. caseiincreased levels of lgAs against Stx1, Stx2, and formalin-killed STEC cells in the colon approximately two-, four-, and threefold, respectively, compared with those of the untreated controls by day 7 after infection. These results suggest that administration ofL. caseistrain Shirota enhances the local immune responses to STEC cells and Stxs and leads to elimination of STEC and thus decreases Stx concentrations in the intestines.