Studies on the Physiological Effects of Microbial Exopolysaccharides
Abstract: The spread of lifestyle-related diseases has become epidemic throughout the world. Impaired glucose metabolism and high levels of plasma cholesterol are risk factors for the development of type 2 diabetes and cardiovascular disease respectively. Soluble dietary fibers from plants, algae and mushrooms have been shown to positively regulate both glucose- and cholesterol metabolism indicating their value in the prevention of lifestyle-related diseases. Some bacteria produce and excrete exopolysaccharides (EPSs) with structures similar to dietary fibers and the aim of the present study was to screen for such EPS-producing bacteria and evaluate some EPSs with regard to their effect on lipid metabolism in mice and glycemic regulation in man. Several lactic acid bacteria were isolated from Indian food products based on their ability to produce EPSs. Their probiotic potential was investigated using several in vitro methods. Two isolates (Weissella cibaria 142 and Pediococcus parvulus AI1) showed interesting probiotic properties that deserves further attention. Two trials were performed to determine the plasma cholesterol and triglyceride levels, fecal bile acid excretion and short-chain fatty acid (SCFA) production in low-density lipoprotein receptor knock-out mice fed a high-fat diet containing EPS for several weeks. In addition, the microbiota composition after ingestion of the EPS-producing lactic acid bacteria Pediococcus parvulus 2.6 and its EPS was investigated. The results show that the EPSs tested did not affect the plasma cholesterol and triglyceride levels or fecal bile acid excretion. However, the production of SCFAs was significantly increased for all but a gel-forming EPS indicating their fermentability by the gut microbiota. The EPS produced by P. parvulus 2.6 greatly affected the composition of the microbiota showing that the EPS was selectively fermented by certain groups in the gut flora; however, the level of bifidobacteria was decreased. The EPS-producing bacterium P. parvulus 2.6 antagonized Enterobacteriaceae below the level of detection but did not affect the overall homeostasis of the microbiota. All of the EPSs investigated provided a bulking effect, which may be beneficial for the maintenance of a well-functioning gut system. The final study aim was to investigate whether the in situ production of EPS renders rheological changes to an oat bran‒based product during fermentation, eventually affecting the viscosity and consequently postprandial glycemia after consumption. An acute meal study was performed using a randomized, single-blind, within-subject design. Eighteen healthy overweight participants consumed four breakfast meals including one reference meal after an overnight fast. The reference meal contained water and white wheat bread, whereas the test meals contained the test beverage (dairy yoghurt, fermented oat bran‒based/milk beverage with and without EPS) and white wheat bread; all the meals contained 50 g of available carbohydrates, but differed in energy and macronutrient composition. All the experimental meals induced lower postprandial glucose responses and had lower glycemic indices compared with the reference meal. The yoghurt and oat bran‒based beverage elicited higher early insulin responses but the insulinemic indices were the same for all meals. The feeling of hunger and palatability were similar for all test products. In conclusion, none of the EPSs induced any change in physiological responses compared with the reference other than the effect observed in the gut (increased SCFA production and changes in the microbiota composition). The EPS-producing bacterium P. parvulus 2.6 had a positive effect on the composition of the microbiota by reducing the number of Enterobacteriaceae. In addition, a new type of convenience food product based on fermented liquid oat bran and milk was shown to attenuate postprandial glycemia as efficiently as yoghurt. This functional food product may help to regulate the postprandial glucose response, which is an important target for the prevention of metabolic disease.
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