Examining Committee
Dr. Lisa Duizer, Chair
Dr. Gisele LaPointe, Advisor
Dr. Steve Cui, Advisory Committee Member
Dr. Keith Warriner, Department Member
Dr. Michael Gaenzle, University of Alberta, External Examiner

TITLE: MICROBIAL UTILIZATION OF ARABINOGALACTAN AND EFFECTS ON HUMAN GUT  MICROBIOTA IN RESPONSE TO BIFIDOBACTERIA SUPPLEMENTATION

ABSTRACT: Arabinogalactan(AG)may improve the abundance of bifidobacteria and the concentration of butyrate in the gut. However, bifidobacteria prefer oligosaccharide utilization, thus most bifidobacteria are unable to utilize AG in pure culture. Therefore, AG utilization by bifidobacteria requires the contribution of other gut bacteria. A better understanding of AG degradation by gut bacteria, and the interaction between bifidobacteria and other intestinal bacteria during AG fermentation is required. This research determined AG-degrading enzymatic functions from Bifidobacterium longum subsp. longum NCC 2705 and Bacteroides caccae ATCC 43185, and investigated metabolic interaction between these strains in the presence of AG by using the simple batch modeling. Moreover, the response of stabilized human fecal microbiota to B. longum subsp. longum BB536 supplementation and fermentation of AG versus starch was compared in the Twin Mucosal Simulator of the Human Intestinal Microbial Ecosystem (TWIN- M-SHIME).

The endo β-1,3/1,6-galactanases are the main enzymes for degrading the backbone and sidechains of AG. These enzyme activities from both B. longum subsp. longum NCC 2705 and Bac. caccae ATCC 43185 strains were hindered by the arabinosyl side chains, which could not be degraded by extracellular α–arabinofuranosidase and/or β-arabinopyranosidase enzymes from both strains. These results show why the breakdown of AG by B. longum subsp. Longum NCC 2705 and Bac. caccae ATCC 43185 is limited in monoculture. In coculture, the growth of B. longum subsp. longum NCC 2705 can be stimulated by the partial carbohydrate breakdown products from AG that were released by Bac. caccae ATCC 43185. However, the growth and metabolism of Bac. caccae ATCC 43185 were inhibited by low pH due to the metabolism of B. longum subsp. longum NCC 2705. Therefore, the cooperation between bifidobacteria and Bacteroides species may not take place in the proximal colon where low pH conditions occur. In the TWIN-M-SHIME model, fermentation of AG significantly increased the concentration of total short chain fatty acids (SCFAs) in all three colon regions, and the abundance of luminal live Akkermansia muciniphila and Faecalibacterium prausnitzii. The combination of B. longum subsp. longum BB536 with AG stimulated butyrate production and increased the abundance of both luminal live and mucosal F. prausnitzii in the transverse colon (TC). These results suggest that combination of a potential prebiotic AG with a probiotic strain B. longum subsp. longum BB536 could exert a beneficial synbiotic effects on human gut health through butyrate production.