Lactobacillus rhamnosus GG-induced probiotic effects require Nox1-generated ROS and SpaC-mediated adhesion

Lactobacilli preparations are marketed for their health benefits and have shown promising efficacy as treatments for intestinal disease. Studies exploring the effect of lactobacilli in the alimentary canals of two common laboratory models, Drosophila and mice, have demonstrated that exposure to lactobacilli positively impacts development, homeostasis, and immune functioning in these systems. However, despite their clinical potential and the existence of laboratory models to investigate them, the nature of the relationship between commensal lactobacilli and host health is poorly characterized. The studies described in this dissertation elucidate some molecular mechanisms behind how probiotic lactobacilli induce salutary effects in the gastrointestinal tracts of flies and mice. Here, we found that Lactobacillus plantarum naturally colonizes laboratory-raised strains of Drosophila and that this bacterium induces cellular reactive oxygen species (ROS) generation in the Drosophila midgut. This cellular ROS generation also activates cell proliferation in the fly midgut. Lactobacilli-induced ROS generation was also shown to occur in cultured mammalian cells, and in the murine intestine. Indeed, Lactobacillus rhamnosus GG (LGG) induces particularly robust ROS generation in mammalian systems. We also established that ROS-generation is dependent on cellular expression of NADPH oxidase (Nox). Lactobacilli-induced ROS generation is significantly diminished in enterocyte specific dNox null Drosophila and in gut epithelial-specific Nox1 null mice. Furthermore, we demonstrated that the LGG pilin adhesion protein, SpaC, is required for LGG-induced ROS generation. An isogenic Lactobacillus rhamnosus GG strain that lacks SpaC (LGGΩspaC) does not adhere to intestinal mucus or murine intestines as well as the wild type strain. SpaC was shown to contribute to the capacity of LGG to induce cellular ROS generation, potentiate ERK MAPK signaling in enterocytes, stimulate cellular proliferation, and protect against radiation-induced injury. Collectively, these data show that commensal lactobacilli found in the alimentary canal lead to beneficial effects in their hosts by inducing cellular ROS generation through a Nox-dependent mechanism. In addition, LGG requires intimate contact with intestinal epithelial cells in order to elicit this ROS induction and its probiotic effects are dependent on bacterial expression of the SpaC pilin protein.