Evaluation of potential probiotic Lactobacillus casei strains

Probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. Lactobacillus casei, due to its high level of consumption, is a probiotic species of particular interest. Significant genetic variability exists within this species with approximately 38% of the genes being variable between L. casei strains. This variability suggests that significant strain-to-strain variation in probiotic efficacy and effects is likely. This thesis describes methods for screening L. casei strains for attributes commonly associated with probiotics through the establishment of in vitro and in vivo models, and utilizes culture-independent methods to characterize the influence of consumption of L. casei on the composition of the gastrointestinal tract (GIT) microbiota of piglets and mice. Strain-specific differences were observed in the ability to survive gastric passage and adhere to the piglet ileum epithelial surface. These results led to the selection of L. casei 32G for further characterization. The ability of L. casei 32G to alter the ileum digesta and epithelial tissue adherent microbiotas was examined in two separate piglet feeding trials. In both studies, significant changes were detected in the dominant genera in both the digesta and tissue samples; however, the specific genera that increased and decreased differed between the two trials. The second piglet trial also examined these alterations in the piglet ileum microbiota over time after the last dose. The results indicated that daily consumption of 32G resulted in significant, relatively short-lived alterations (hours) to the composition of both the digesta and Peyer's patch microbiotas. The influence of 32G dose on the ability of this strain to alter the composition of the GIT microbiota was evaluated in mice. The results demonstrated that dose-dependent changes occur in the cecum microbiota of mice upon administration of L. casei 32G and that the lowest dose examined (10 6 CFU/day for seven days) had the most dramatic impact on this microbiota. In conclusion, L. casei 32G was selected as the strain examined with the greatest probiotic potential and was shown to cause restructuring of the GIT microbiotas in both the piglet and mouse models.