Polysaccharides intestinal bacteria

A06A C Bulk Producers Within this group there are fibres, which consist of plant polysaccharides and lignins. Fibres can be divided into non-soluble and gelforming. The non-soluble fibres are resistant against any effect of the intestinal bacteria, while the... 

Table 1. Species of intestinal bacteria which can degrade various indigestible polysaccharides (, 10) ...

There are a number of other polysaccharides in foods. Collectively they are known as non-starch polysaccharides, the major components of dietary fibre (section 7.3.3.2). Non-starch polysaccharides are not digested by human enzymes, although all can be fermented to some extent by intestinal bacteria, and the products of bacterial fermentation may be absorbed and metabolized as metabolic fuels. The major nonstarch polysaccharides (shown in Figure 4.8) are ... 

Non-starch polysaccharides A group of polysaccharides other than starch which occur in plant foods. They are not digested by human enzymes, although they may be fermented by intestinal bacteria. They provide the major part of dietary fibre. The main non-starch polysaccharides are cellulose, hemicellulose (insoluble non-starch polysaccharides) and pectin and the plant gums and mucilages (soluble non-starch polysaccharides). 

Van Laere, K. M. J., Hartemink, R., Bosveld, M., Schols, H. A. Voragen, A. G. J. (2000). Fermentation of plant cell wall derived polysaccharides and their corresponding oligosaccharides by intestinal bacteria. Journal of Agriculture and Food Chemistry, 48, 1644-1652. 

Fiber components are the principal energy source for colonic bacteria with a further contribution from digestive tract mucosal polysaccharides. Rate of fermentation varies with the chemical nature of the fiber components. Short-chain fatty acids generated by bacterial action are partiaUy absorbed through the colon waU and provide a supplementary energy source to the host. Therefore, dietary fiber is partiaUy caloric. The short-chain fatty acids also promote reabsorption of sodium and water from the colon and stimulate colonic blood flow and pancreatic secretions. Butyrate has added health benefits. Butyric acid is the preferred energy source for the colonocytes and has been shown to promote normal colonic epitheUal ceU differentiation. Butyric acid may inhibit colonic polyps and tumors. The relationships of intestinal microflora to health and disease have been reviewed (10). 

Di-, oligo-, and polysaccharides that are not hydrolized by a-amylase and/or intestinal surface enzymes cannot be absorbed, and they reach the lower tract of the intestine, which, from the lower ileum onward, contains bacteria. Because bacteria contain a larger variety of saccharidases than humans, they can use many of the remaining carbohydrates. 

Dietary fiber, composed principally of polysaccharides, cannot be digested by human enzymes in the intestinal tract. In the colon, dietary fiber and other nondi-gested carbohydrates may be converted to gases (H2, CO2, and methane) and short-chain fatty acids (principally acetic acid, propionic acid, and butyric acid) by bacteria in the colon. 

The focus on bacterial fermentation of components of dietary fiber from conventional sources such as bran should not prevent us from considering other types of carbohydrate which may also be fermented by colon bacteria. For example, the host itself produces a variety of polysaccharides which reach the colon. These include mucins from the secretions which lubricate the intestinal tract and mucopolysaccharides from slonghed epithelial cells. It is difficult to obtain precise information concerning how much of this material is produced daily, but the amount could easily be comparable to the amount of polysaccharide which is ingested in the diet. Some of these host-produced polysaccharides, particularly the mucopolysaccharides, are readily utilizable by some of the same bacterial species which ferment plant polysaccharides ( ). 

From these examples, it can be seen that studies of the effect of diet on the intestinal flora must take into account the availability of a variety of polysaccharides. Information about in vitro fermentation of different polysaccharides by colon bacteria can help us to make predictions about what sorts of carbohydrate may be utilizable in vivo, but this information raises still other questions. To see this, consider some of the results of surveys of polysaccharide fermentation by numerically predominant species of colon bacteria (, 10). Some of these results are summarized in Table 1. One thing that is immediately apparent from Table 1 is that the most versatile fermenters of polysaccharides are Bacteroides species. Moreover, many of these species can ferment several types of polysaccharides. 

Endotoxins. Name for bacterial toxins which, in contrast to the exotoxins, are not secreted by living bacteria but are released from the cells by autolysis (e.g. in the intestines). In the case of classic E. they consist of the thermostable lipopolysaccharide (LPS-) fraction of the cell membrane anchored on the outer membranes of Gram-negative bacteria. The LPS consists of lipid A, the chain polysaccharide, and the O-specific chain lipid A is responsible for the toxic action of LPS. E. are found in all Enterobacteriaceae, e.g.. Salmonella (typhus), Shigella (dysentery), and many other Gramnegative pathogens. In the host organism E. stimulate mediators (cytokines) of the immune system. One of... 

Severe nutritional deficiencies, X-irradiation, the administration of antifolic (see Fig. 4-48) or bactericidal agents allow some bacteria of the intestinal lumen to pass through the mucosa and enter the bloodstream to cause septicemia. Furthermore, the endotoxins normally present in the intestinal lumen enter the bloodstream under certain pathological conditions and transform reversible shock into irreversible shock. All gram-negative bacteria produce endotoxins—large molecules with a molecular weight of 1,000,000 formed of a complex of proteins, lipids, and polysaccharides. 

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