Branched SCFAs Gases H2 CO2
Amines + ammonia Phenols Bacterial biomass absorption, pH, epithelial cell metabolism, motility and bowel habit and colonization resistance, in addition to providing products that are absorbed and reach the liver and peripheral tissues." In addition to fermenting the nondigestible nutrients, the colonic microflora also contributes to the metabolism of endogenous compounds and xenobiotics: the primary bile acids are transformed, via the 7D-dehydroxylation, into secondary (deoxycholic and lithocholic acids), and, eventually, tertiary (ursode-oxycholic acid) bile acids: biluribin and biliverdin are metabolized to produce the uro- and stercobilinogens: miscellaneous food components like alkaloids, flavonoids, glucosinolates, saponins, terpenes, coumarins, sulfur-containing compound have either beneficial or deleterious effects in the human body are hydrolyzed or reduced.7 Furthermore, the colonic bacteria produce some of the B vitamins and vitamin K, as well as folic acid, which may contribute to the body's pool of nutrients.
The human adult colon has a length of ±1.5 m and a surface of ±1.3 m2. Its content has a pH that ranges from 5.8 in the cecum up to 6.5 in the recto-sigmoid (Figure 2.4). Its content represents ±0.25 kg of mass composed mainly of bacteria (±1011/g dry matter) sitting on particles of food residues surrounded by a matrix of glycoprotein and exfoliated cells. In the feces, several hundreds of bacteria have been identified, but some 30-40 species belonging to five or six genera account for 99% of the biomass. Table 2.4 lists the predominant bacteria so far isolated and identified in human feces. Most of these bacteria are nonsporing anaerobes.
The recent development and validation of molecular methodologies (especially the fluorescent in situ hybridization technique using specific ribosomal RNA [rRNA] sequences known as the FISH method in Chapter 9, Section 9.3) has initiated very active research on the composition of human fecal flora. The experiments so far reported have already revealed the existence of yet unknown/noncultivable genera and species. The number of bacteria expressed as colony-forming units (cfu) of the different species range from 102 up to 1010 per gram of fresh feces. Each species has its own nutrition requirements and produces a specific pattern of metabolic end products. The colonic microflora is acquired at birth and during the early days, weeks, and months after birth. It is recognized today that the composition of the first colonic microflora is a key element in the development of the immune system early in life. Furthermore, the maintenance of a well-balanced composition throughout life is considered an important factor for well-being and health. There are differences between the different segments of the colon, especially between the cecum and ascending colon on the one hand, and the descending colon and sigmoid colon on the other. The right part of the colon is relatively rich in carbohydrates that are fermented by saccharolytic species to yield principally linear SCFAs, H2, and CO2. The content is well moistened and relatively acidic, and so bacteria proliferate rapidly. The left part of the colon is carbohydrate- and water-depleted. Protein breakdown and amino acid fermentation by proteolytic bacteria become more dominant; this produces more branched-SCFAs, H2, CO2, CH4, phenols, and amines. The pH is closer to neutrality (±7) and proliferation of bacteria is slower. The durations of residence in the different segments of the colon are also different — between 6 and 16 h, and 12 and 36 h in the right and left side, respectively. The average transit time in the large bowel is ±60 h, with a range of 23-168 h.11
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