Date of this Version
Rattanaprasert M. 2015. Ph.D. dissertation. University of Nebraska-Lincoln, Lincoln, Nebraska. Efficacy of galactooliosaccharide (GOS) and/or rhamnose-based synbiotics in enhancing ecological performance of Lactobacillus reuteri in the human gut and characterization of its GOS metabolic system
Probiotic L. reuteri has potential to produce antimicrobial compounds and secrete immunosuppressive factors. These metabolic attributes could benefit the human host by providing colonization resistance (competitive and metabolic exclusion) against enteropathogens and mitigating inflammation. As metabolically active cells are fundamental to such probiotic properties, synbiotic approaches that supply L. reuteri with a source(s) of carbon, energy, and/or external electron acceptor for cell growth in the gut environment could therefore prompt the probiotic to engage in beneficial activities. In this study, the efficacy of GOS and/or rhamnose-based synbiotic approaches in promoting colonization persistence and metabolic activity of L. reuteri was evaluated. A single blind, randomized, crossover, placebo-controlled human trial revealed that daily administration of the L. reuteri DSM 17938 (5 x 108 cells)/GOS (1g)/rhamnose (1g) synbiotic combination significantly stimulated metabolic activity of the probiotic strain in the human gut. This positive outcome presumably results from the ability of L. reuteri to metabolize GOS as a carbon and energy source, while utilizing rhamnose as an external electron acceptor for redox balance. In contrast, neither GOS (2g) nor rhamnose (2g) alone could exert such stimulatory effect. In addition, after the synbiotic administration was terminated, the extended supplementation of the carbohydrates without L. reuteri did not appear to improve the persistence of the probiotic in the gut.
Genetic characterization of GOS metabolic machinery disclosed that GOS metabolism in L. reuteri is inducible and is under carbon catabolite repression (CCR). The metabolic system relies on LacS permease and a second transporter to import diverse GOS species into the cytosol. Then, two b-galactosidases, LacA and LacLM, sequentially break down GOS oligosaccharides as well as concertedly hydrolyze GOS disaccharides. The system is regulated by repressor protein LacR and fully activated only in the presence of inducer lactose and in the absence of glucose (i.e., a preferred carbon source). Furthermore, a growth advantage only the wild type strain, but not the GOS metabolic gene-deficient mutant, gained in the GOS-enriched murine gut suggests that the GOS metabolic system be operational in the gut environment.
Advisors: Robert W. Hutkins and Jens Walter