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GRK 1482 Jahrbuch 2011-2014

Abstract After ingestion food-borne pathogens like Salmonella Typhimurium (S. Typhimurium) face a densely populated, environment in the gas- trointestinal tract and have to establish metabolic niches to gain an advantage over commensal bacteria in the competition for nutrients. Nevertheless these pathogens are able to overcome the colonization resistance of the gut, replicate and invade the mucus barrier reaching the epithelial surface. Still less is known about the mechanisms Sal- monella prevails against the colonization resistance and establishes its metabolic niche in the gut. Introduction After ingestion pathogenic bacteria are confronted with a densely populated, competitive environment [1], since the human gastrointestinal tract (GIT) is inhabited by about 1014 commensal bacteria [2]. Commensal bacteria are fermenting ingested food, thus providing essential nutrients such as short- chain fatty acids and vitamins for their host [3]. Pathogenic bacteria like Salmonella Typhimurium have to compete for nutrients with commensal bacteria and have developed strategies to over- come the colonization resistance of the gut and the mucus barrier in order to reach the epithelium and cause infection [4]. In the stomach and small intes- tine available nutrients are for example ingested carbohydrates and proteins as well as monosac- charides, fatty acids, amino acids, cholesterol, di- and tripeptides [3]. Polysaccharides being indiges- tible for the host are passed to the large intestine where they are degraded by mainly anaerobic mi- croorganisms [5]. Since pathogens have to compe- te for nutrients with the commensal flora they have established metabolic pathways enabling the use of alternative carbon sources like L-fucose [1], pro- pionate, 1,2-propanediol, ethanolamine, melibiose, L-ascorbate and D-xylulose [6], available in the gut lumen to overcome the depletion of carbon sources by commensals. Between the gut lumen and the epithelial surface a physical barrier has to be over- come since the epithelial surface is covered by gly- cocalyx and mucosa protecting epithelial cells from harmful bacteria. The mucosa covers the glycoca- lyx and is built of two mucus layers in the stomach and the colon, and one layer in the small intestine [7]. Commensal bacteria can be found in the loose outer mucus layer but not in the compact inner mucus layer [8]. Glycans of the mucus layer serve as attachment sites for bacteria and as rich feedstock, due to secretion of glycan degrading enzymes by commensal bacteria [8]. At the mucosal wall, sugars like mannose, galactose, fucose and glucose are available, which are components of glyco- proteins of mucins often released by commen- sal degradation [7], [6], [9]. To escape the densely populated regions of the gut most pathogenic bac- teria are expressing flagella to actively move into the compact mucus layer towards the epithelial surface [10], [1]. ASSOCIATED FELLOWS Page 76 | GRK Progress Report 2011-2014 Lena Staib (M.Sc.) Microbial Ecology PhD 4/2 Microbial metabolism relevant for gut niche occupation