A comparative study of two Lactobacillus fermentum strains that show opposing effects on intestinal barrier integrity: a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy, Massey University Manawatu, New Zealand.

The Lactobacillus species can exert health promoting effects in the gastrointestinal tract (GIT) of humans through several mechanisms, which include pathogen inhibition, maintenance of microbial balance, immunomodulation and enhancement of the GIT barrier function. However, different strains of lactobacilli can evoke different responses in the host and not all strains of the same species can be considered health promoting. Two strains identified as Lactobacillus fermentum, namely AGR1485 and AGR1487, isolated from human oral cavities, exhibit opposing effects on intestinal barrier integrity. Studies have shown that AGR1485 maintains trans-epithelial electric resistance (TEER), a measure of GIT barrier integrity, across Caco-2 cell monolayers, while AGR1487 decreases TEER by 12 hours. This work aimed to test the hypotheses that the varying effects shown by these two L. fermentum strains are related to phenotypic differences between the two strains and are mediated by the interaction of secreted and/or cell-associated bacterial components with the GIT epithelial layer. Differences in metabolic events that occur during the various phases of growth in bacteria can impact not only cellular structure and secreted molecules, but may also affect their interactions with the intestinal epithelial cells. TEER assays were conducted to investigate if variation in bacterial secreted molecules and cell wall components associated with various phases of microbial growth can affect Caco-2 cell TEER. The effect on Caco-2 cell TEER caused by both strains was independent of bacterial growth phase. To test the hypothesis that it is the bacterial structural and/or secreted components that influence Caco-2 cell TEER, assays were conducted with live versus UV-killed bacteria on Caco-2 cells. ... The results reported in this dissertation have added to the knowledge that the two strains of L. fermentum AGR1485 and AGR1487 show differences in their genome size and in their phenotypic characteristics. In addition, these bacteria utilise both cell surface and/or secreted metabolites through multiple mechanisms to modulate host response. In the future, identification of specific bacterial effector molecules that influence host response will be a major step towards understanding strain-specific characteristics shown by Lactobacilli.