Commensal Bacteria of the GI Tract: Benefits of Probiotic Supplementation (Post 2)

MICROBIAL CONTENT OF THE GI TRACT

The human colon is about 5 feet long and has a volume of approximately 500 ml, or about 1 pint. In healthy adult humans, approximately 1.5-2 liters (1 liter ~1 quart) of chyme (mixture of food, liquid, and stomach and intestinal secretions) are emptied from the small intestine into the colon each 24-hour period.¹ The major portion of chyme (water) is absorbed by the colon into the bloodstream. Much of the solid portion of colonic chyme consists of undigested carbohydrates. The colonic microflora, almost all of which are anaerobic (either strict or facultative), ferment these sugars to short-chain fatty acids (mostly acetate, propionate and butyrate) and to gases such as carbon dioxide, methane and hydrogen.² Other matter (food particles, mucus and other molecules) are broken down to various byproducts which include polyamines, amino acids, growth factors, antioxidants and vitamins.³

Prior to birth the intestines contain no microorganisms. During the birth process vaginal microbes are ingested, and within 1-3 days large numbers of enterobacteria and streptococci are established. Shortly thereafter, depending upon the type of food ingested, the intestines become populated with various genera of bacteria (Figure A). Within 3-4 days of birth, colons of breast-fed infants become dominated by bifidobacteria species while enterococci predominate in bottle-fed infants. After weaning, bifidobacteria decrease as colons begin to adopt a more adult profile, mostly based on diet.^{2 ,4}

The adult colon’s contents are about 35-50% bacteria, which accounts for the majority of the bacteria found in the body. In fact, the population of microbes in the colon is estimated to be 10 times that of the total number of cells that make up the human body (between 40 and 55% of the dry weight of feces is composed of bacterial cells).^{2 ,5 ,6} Genera that are prominently represented include Lactobacillus, Bacteroides, Peptococcus, Peptostreptococcus, Bifidobacterium, Clostridium, Fusobacterium, Eubacterium, Escherichia and Veillonella. Bacteroides, which includes species that utilize a wide range of polysaccharides as nutrient sources, are most numerous and can comprise more than 30% of the total colonic population.^{2 ,7}

More than 800 species (>7,000 strains) of bacteria are estimated to inhabit the colon. These include species that are commensal; that is, those that live off the gut but cause no harm, and those that are potentially pathogenic (cause disease).⁵ As mentioned above, many colonic microbes provide beneficial effects and are necessary for proper colon function. Some of these effects include absorption of water, electrolytes and other nutrients, synthesis of certain B vitamins and vitamin K, and digestion of certain carbohydrates (dietary fibers).^{7 ,8 ,9}

The short-chain fatty acids (SCFAs) produced by fermenting carbohydrates can be used by intestinal microbes as an energy source. SCFAs stimulate colonic blood flow and fluid and electrolyte uptake. More than 95% of the SCFAs produced by colonic microbes are absorbed by the colon. SCFAs contribute to normal bowel function and prevent pathology² and aid in absorption of calcium and magnesium by the colon.¹⁰ The anti-inflammatory activities of SCFAs are discussed below in the Inflammatory Bowel Diseases section.

Other benefits include aiding intestinal maturation in neonates, proper mucosal barrier fortification and stimulation of immune function, and prevention of deleterious effects from pathogenic microbes.^{3 ,11 ,12} This function is performed by various means, including inhibiting adherence of pathogenic microbes to the colonic mucosa, thus reducing their ability to colonize and “set up shop;”^{13 ,14} synthesis of antipathogenic substances; competition for nutrients required by pathogens; modification of toxins or toxin receptors; reduction of permeability of the gut; and stimulation of non-specific and specific immune responses to pathogens.^{13 ,15 ,16 ,17} Large amounts of hydrogen gas produced by fermentation are beneficial in speeding the transit of colonic contents, which helps reduce intestinal uptake of toxic bacterial metabolites.¹⁸

Although most recent research has focused on the colon and its microflora, consideration must be given to the ileum and its microbial population, primarily because of the differences in absorption of nutrients by these organs. The small intestine is approximately 20 feet long. As food traverses its length, the pH rises as the distance from the stomach increases until, upon reaching the ileum, a much more favorable environment exists for viable microflora. Indeed, the microbial makeup of the ileum is very similar to that of the colon, except that the numbers, though tremendous, are not as high as those of the colon.

As mentioned earlier, the small intestine absorbs most of the nutrients digested in the GI tract. This is significant when considering the microbial content of the ileum. For example, vitamin K is synthesized by commensal microbes and absorbed by the ileum.^{19 ,20} Some strains of lactobacilli produce lactase, the enzyme that degrades lactose. Thus, these bacteria can provide benefit to people who are lactose-intolerant by breaking down lactose to glucose and galactose, which can then be absorbed by the ileum.

When one considers beneficial intestinal microflora, lactobacilli and bifidobacteria are the two most common genera that come to mind. However, specific species and strains of other genera, even those generally considered to be pathogenic, have been shown to provide healthful benefits. Some examples include Enterococcus, Clostridium and Escherichia.^{21 ,22} When an imbalance in microbial populations occurs that favors pathogenic species (often the result of antibiotic therapy) harmful effects such as diarrhea and infectious diseases of the GI tract are often seen, and chronic conditions such as inflammatory bowel diseases and even colon cancer may result.

Restoration of intestinal health usually results when normal microbial populations are regained. An increasingly popular means by which this is accomplished is with the ingestion of live microbial cultures, generally in food such as milk or yogurt, or in dietary supplements. Food and/or dietary supplements that provide live beneficial microbes are called “probiotics.”²³ “Probiotics” is also commonly used in the literature as a general term to refer to beneficial microbes in the colon, even though this is technically incorrect. For purposes of accuracy, this paper will restrict the term to dietary products that contain beneficial microbes.

Much of what transpires in the GI tract remains a mystery. However, recent research has shed much light on the interplay between the intestinal structures and the populations of microbes that inhabit them. Although these microbes can use various materials such as mucus from the lining of the intestines as food, they generally “eat” what we eat; that is, they use the foodstuff that is presented to them from the stomach. Much of what makes it to the ileum and colon is dietary fiber, or non-digestible polysaccharides, such as cellulose. In fact, the study of the lower GI tract and its microbial content is in very large part the study of dietary carbohydrates and the means by which the body uses these nutrients in attaining and maintaining good health.

Polysaccharides consist of single molecules of sugars (monosaccharides) that are linked together into chains. Although the GI tract can apparently absorb a few oligosaccharides, or short sugar chains,²⁴ generally, polysaccharides must be broken down into monosaccharides before absorption can occur. Humans can digest carbohydrates such as sucrose or starches because the monosaccharides are hooked together by alpha (α) linkages, which we can hydrolyze with certain digestive enzymes. However, other polysaccharides such as cellulose, which makes up a large portion of plant tissues, are made of monosaccharides that are bound together with beta (ß) linkages. We produce only one enzyme that can break this kind of linkage (lactase, which digests the ß-linked disaccharide lactose, or milk sugar). Almost all other ß-linked polysaccharides pass through the digestive tract until they reach the ileum and colon, where bacteria do possess enzymes that break ß linkages. There, polysaccharides are reduced to monosaccharides that are then either absorbed or fermented to short-chain fatty acids (SCFAs) and various gases.²

Not all intestinal microbes are equal regarding the digestion of these plant polysaccharides. Bacteroides species are major players in the breakdown of ß-linked plant polysaccharides in the colon. They have been shown to be effective in hydrolyzing these sugars from various samples such as pectin, gum tragacanth, guar gum, gum Arabic, gum ghatti and larch arabinogalactan. Other bacteria known to ferment some of these sugars included those from Peptostreptococcus, Bifidobacterium, Ruminococcus and Eubacterium genera.^{25 ,26}

Bacteroides thetaiotaomicron

Bacteroides thetaiotaomicron is a species of bacterium that is found in the ileum and the colon. It is called a glycophile (lover of sugar) because of its ability to digest an enormous variety of carbohydrates. The genome of B. thetaiotaomicron has been completely sequenced, and it contains the largest number of genes involved in metabolizing carbohydrates of any bacterium known. For example, 172 glycosylhydrolases have been identified (226 are predicted) compared to 39 in Bifidobacterium longum and only four in Pseudomonas aeruginosa.

The 172 enzymes together appear to be able to cleave most glycosidic bonds known in nature. Approximately 11% of these glycosylhydrolases are present in the outer cell membrane or are excreted. So not all of the carbohydrates acted upon by this microbe are ingested and fermented, but rather are hydrolyzed into oligosaccharides and monosaccharides outside the cells. Thus, B. thetaiotaomicron may function as a foundation species that breaks down a large variety of polysaccharide bonds and supplies simple sugars to the intestinal epithelial cells for absorption into the bloodstream, or to other intestinal microbes that are not capable of digesting intact plant polysaccharides.^{7 ,27}

B. thetaiotaomicron does not produce a glycosylhydrolase until it is exposed to the specific carbohydrate to be digested. On the other hand, if it needs a specific sugar that is not being supplied in the host’s diet, B. thetaiotaomicron has the ability to demand that sugar’s production from intestinal cells. For example, in studies with gnotobiotic mice, B. thetaiotaomicron was able to evoke ileal expression of the polysaccharide, Fucα1,2Galß-glycan, from which the bacterium can scavenge the sugar, fucose. When enough fucose is harvested, the bacterium signals the intestinal cells to shut down production of this particular polysaccharide. This phenomenon is thought to be common among other intestinal bacteria.²⁸

Sugars are also called Glyconutrients.

Glyconutrients are used for the process called cell to cell communication.

Mannatech is the worlds authority on Glyconutrients. Mannatech are based in Coppell Texas