Bacteria Present in Human Gastrointestinal Tract
The microial flora in the GI tract of human infants begins establishment immediately after birth of child. Anaerobic bacteria can be recovered from fecal specimens after birth . The vaginal and fecal flora of the mother, and also the environment (air and food, for instance) are important sources of bacteria. Changes in the diet can modify the microbial composition in the colon . For example, infants fed breast milk have mainly Bifidobacteria in their gut, but infants fed formula have more complex microfl ora which are predominantly Bacterioides, Bifidobacteria, Clostridia and Streptococci
In the GI tract the large intestine is the most colonized area and can harbor up to 1012 bacteria per gram of waste contents with hundreds of species represented . The large intestine contains mostly strict anaerobes and they may be categorized as bacteria that are either harmful or benefi cial . The pathogenic effects of harmful bacteria include diarrhea, systemic infections, liver damage and carcinoma. In contrast, the beneficial bacteria promote the host’s health by stimulating the immune system, synthesizing vitamins, inhibiting the growth of harmful bacteria and improving digestion and absorption of essential nutrients . Furthermore, the microflora in the large intestine derives a proportion of their energy from dietary components that have not been degraded in the upper intestinal tract and reach the large intestine. These dietary components consist mainly of polysaccharides, oligosaccharides, proteins, peptides and glycoproteins that are fermented by the gut bacteria and subsequently produce short chain fatty acids (SCFA) as their major end products; the majority of these SCFA are acetate, propionate and butyrate .
Probiotics
Probiotics are live microorganisms which when consumed may confer a health benefi t to the host . Certain species of Lactic Acid Bacteria (LAB) and Bifi dobacteria spp. are commonly used in the manufacture of probiotic products because of their well-known benefi cial eff ect to host health and they are Generally Recognized as Safe (GRAS) . Many studies have shown that probiotics can stimulate the immune system decrease serum cholesterol, alleviate lactose intolerance, decrease diarrheal incidence, control infections, act as antibiotics, suppress tumors and protect against colon/bladder cancer.
Role of probiotics in lactose intolerance
For most mammals, including humans, lactase activity declines aft er weaning; however, in some humans, lactase activity persists at a high level throughout adult life due to the development of lactase persistence . Th is lactase persistence among the population of the world varies depending on ethnicity and habit of consuming dairy products; consequently, the northern European population has as little as 2% of the population defi cient in lactase. Th e lactase defi ciency is prevalent in Hispanic people (80%), black and Ashkenazi Jewish people (60 to 80%) and almost 100% of Asian and Native American Indian people.
In persons lacking lactase, when lactose reaches the large intestine, it is metabolized by the colonic microfl ora to produce methane, carbon dioxide, and hydrogen and altering the osmotic balance in the colonic lumen, causing symptoms such as cramping, diarrhea, flatulence, and abdominal bloating. However, a number of human studies have shown that high-lactose milk products supplemented with starter cultures containing Lactobacilli and/or Bifidobacteria can be tolerated by lactose-intolerant individuals, possibly because these fermented products contain the microbial β-galactosidase which functions in the small intestine to support lactose hydrolysis. Additionally, it has been demonstrated that in mice Streptococcus thermophilus or Lactobacillus casei subsp defensis are able to hydrolyze lactate during transit through the gut. Independent of easing lactose intolerance, probiotics also seem to relieve some gastrointestinal complaints such as flatulence or diarrhea most probably by their influence on the gut microflora.
Probiotics effects on diarrheal diseases
Infectious diarrhea caused by bacteria or viruses The World Health Organization (WHO) estimates 8.1 million deaths occur yearly in children (<5 years of age) with diarrhea accounting for 14% of those deaths . In addition to causing mortality, diarrhea elicits serious long-term effects with multiple episodes and persistent diarrhea affecting growth and cognition. Rotavirus is the most common cause of diarrhea in children. The beneficial effects of certain probiotic strains on preventing rotavirus-caused diarrhea have been studied. In one study, human infants aged 5 to 24 months who were admitted to a chronic medical care hospital were randomized to receive a standard infant formula or the same formula supplemented with Bifidobacterium bifidum (1.9×108 CFU/g) and Streptococcus thermophilus (1.4×107CFU/g) for 14 days. The results demonstrated significantly lower numbers of children experiencing diarrhea who received the supplemented formula compared to un-supplemented control groups. Other beneficial effects recorded from clinical studies include shortening the duration of diarrhea by as much as 1.5 days and less shedding of rotavirus . The European Society for Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) and the European Society for Pediatric recommend that only probiotic strains with proven clinical efficacy and in appropriate dosages be used as an adjunct to rehydration therapy for the management of children with acute diarrhea. Proven clinical efficacy was determined from formal systematic reviews of the literature; treatments were highly recommended if there was strong evidence from more than one systematic review of well-designed randomized control trials that a probiotic was more effective than a placebo. It was determined that probiotics L. reuteri (ATCC 55730), L. acidophilusLB, S. boulardii and a mixture of S. thermophilus, L. acidophilus and L. bulgaricus significantly reduced the duration of diarrhea when compared with placebo, particularly in rotaviral gastroenteritis . Effects of probiotics were also determined to be dose dependent dose-dependent, that is the effects were greater for doses higher than 1010 to 1011colony-forming units . The World Gastroenterology Organization (WGO) included L. casei DN-114 001 in their evidence-based recommendation and pointed out that timing of administration (at the very first sign of illness) is considered highly important.
Antibiotic associated diarrhea Many patients taking antibiotics that disturb the gastrointestinal flora experience diarrhea as a side effect . Symptoms range from mild and self-limiting to severe, and this antibiotic-associated diarrhea (AAD) is an important reason that patients refuse to take antibiotics or curtail their treatment regimen. Many studies indicate the ingestion of certain probiotic strains, primarily Lactobacilli either alone or in combination with other genera, before and during antibiotic treatment reduces the frequency, duration and severity of antibiotic induced diarrhea.
Lowering of cholesterol
There have been several mechanisms proposed for the purported cholesterol lowering effects of probiotics, including deconjugation of bile acids by bile-salt hydrolase enzymes of probiotics, assimilation of cholesterol by probiotics , co-precipitation of cholesterol with deconjugated bile , cholesterol binding to cell walls of probiotics, incorporation of cholesterol into the cellular membranes of probiotics during growth, conversion of cholesterol into coprostanol and production of short-chain fatty acids upon fermentation by probiotics in the presence of prebiotics.
Numerous studies have demonstrated convincing cholesterol-lowering effects of probiotics in both animals and humans
Cancer Prevention
Yogurt and other fermented milk products have been promoted as cancer preventing, especially for colon cancer . Epidemiological studies suggest that colon cancer is associated with a high-fat diet because of the large international variations in rate are strongly correlated with apparent per capita consumption; the high fat diet potentially stimulates bile acid turnover and can lead to an increase of bile acids in the colon which affects the metabolism of the bacterial flora. The bile acids sodium cholate and sodium chenodeoxycholate were studied as promoters of colon carcinomas by comparison of conventional rats to germ free rats and these researchers concluded that indigenous microflora in the intestine could produce enzymes such as glucuronidase, nitroreductase and azoreductase which can convert pro-carcinogens into carcinogens. This has prompted research on the use of probiotic bacteria to reduce the risk of colon cancer. However, the factors involved in the initiation and promotion of colon cancer are separated in time from actual tumor development, making it difficult to find definitive indicators of usefulness of probiotics. Brady et aL.analyzed animal studies over a 10 year period that looked most directly at the relationship between probiotic consumption and colon cancer development and found that apparently probiotics do have an inhibitory effect on the development of precancerous lesions and tumors in animal models. Several mechanisms have been proposed for the anti-cancer effects of probiotics including preventing the growth of bacteria that produce enzymes responsible for converting pro-carcinogens into carcinogensinhibition of tumor growth by metabolites of the Lactobacilli stimulating the immune system , and production of tumor necrosis factor by macrophages .
For probiotics to be effective they must be alive in the food or supplement as well as being able to survive the harsh conditions of the GI tract while still maintaining activity . A multitude of factors affect viability and survivability including the strain of bacterium, pH of the food or supplement, nutrients in the food or supplement and heat treatment . A myriad of methods have been developed to increase survivability and viability of the probiotics ranging from pre-exposure to sublethal stresses, use of immobilized cell technology, microencapsulation, genetic modification, combining different synergistic strains, to incorporation of nutrients and prebiotics to the matrix .
Prebiotics
Prebiotics are employed to promote both beneficial bacteria which are already established in the colon as well as externally administered probiotic bacteria. The original definition of prebiotics was that they are food ingredients which are indigestible in the upper GI tract and reach the colon to beneficially influence the host by selectively promoting the growth and/or activity of certain bacteria in the colon. Given the large number of bacterial strains present in the GI tract, some of which are non-cultivable, the definition was revised to “a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health”. Consumers often take in moderate levels of prebiotics naturally from many fruits and vegetables including leeks, Jerusalem artichokes, chicory, onion, garlic, banana, and asparagus, but the levels of prebiotics in these food sources are generally too low to exhibit any significant effect on the composition of intestinal microflora . Thus, prebiotics are commercially extracted and concentrated from fruits and vegetables through the hydrolysis of polysaccharides from dietary fibers or starch, or through enzymatic generation. Prebiotics are mixtures of indigestible oligosaccharides, except for inulin which is a mixture of fructooligo- and polysacharides. All currently known prebiotics are carbohydrates, and there are many different carbohydrates marketed world-wide as prebiotics, the only four that are well supported by good quality data from human trials are the fructans (inulin and fructo-oligosaccharides), galacto-oligosaccharides, and the synthetic disaccharide, lactulose.
Fructans
A fructan is composed of fructose polymers which are generally linked to the moiety of a terminal glucose . Fructans are resistant to enzymes in the digestive tract of human and pass through the upper portion of the human GI tract, reach the colon and are fermented by colonic microflora producing SCFA .
Inulin
Inulin is a naturally occurring storage carbohydrate commonly found in leeks, onions, wheat, asparagus, garlic, Jerusalem artichoke and chicory . Most of the inulin commercially produced is from Jerusalem artichokes and chicory . The effect of daily intake of 10 g inulin on fasting blood lipid, glucose and insulin levels in healthy middle-aged men and women with moderately raised total plasma cholesterol and triacylglycerol levels was studied in a double-blind randomized placebo-controlled parallel study in which subjects received either inulin or placebo for a period of 8 weeks . Fasting blood samples were collected before the supplementation period and at weeks 4 and 8, with a follow-up at week 12. Compared with baseline values, insulin concentrations were significantly lower at 4 weeks in the inulin group, and there was a trend for triacylglycerol values to be lower in the inulin group at 8 weeks returning to baseline concentrations at week 12 . Nair reviewed the literature on functional aspects of inulin and reported that several animal and human studies have shown inulin to promote optimal digestive health, positively influence lipid metabolism, decrease the risk of osteoporosis by increasing calcium absorption, as well as reduce the risk of colon cancer, breast cancer, and tumor growth.
Fructo-oligosaccharides
Fructo-oligosaccharides (FOS) are composed of a mixture of oligosaccharides consisting of glucose linked to fructose units by β-(1,2) links with a degree of polymerization (DP) between 1 and 5. They occur naturally in several plants such as asparagus, wheat, Jerusalem artichokes, and rye. Onion is especially rich in FOS which ranges from (25 to 40% of dry matter . The commercial production of FOS is mainly based on two processing methods which are either a continuous process using immobilized cells in calcium alginate gel or enzyme on an insoluble carrier or a batch conversion of sucrose by fungal fructosyltranserase . In the human colon the FOS are completely fermented mostly to lactate, SCFA (acetate, propionate and butyrate), and gas . The FOSs stimulate Bifidobacterial growth and suppress the growth of potentially harmful species in the colon . Other effects of FOS include a decrease in fecal pH, an increase in fecal or colonic organic acids, a decrease in fecal bacterial enzymatic activities and a modification in fecal neutral sterols. They have been demonstrated to enhance magnesium absorption in humans and, in animal models, have demonstrated reduction in colon tumor development due to enhancement of both colon butyrate concentrations and local immune system effectors .
Galacto-oligosaccharides
Galacto-oligosaccharide (GOS) is a collective term for a group of carbohydrates composed of oligo-galactose with some lactose and glucose, and which are produced commercially from lactose by β-galactosidase . There are oligosaccharides that resemble GOS naturally found in human milk which may be one of the factors that protect breast fed infants from gastrointestinal pathogenic bacteria. Several animal studies, mainly in rats, evaluated GOS and have generally been shown to promote growth of the beneficial Bifidobacteria . The effect of feeding a diet containing GOS with or without Bifidobacterium breve (administered in the drinking water) was investigated in rats colonized with a human fecal microflora . The GOS or GOS plus B. breve, given for 4 weeks, induced increases in cecal concentration of total anaerobic bacteria, Lactobacilli and Bifidobacteria, and decreases in numbers of enterobacteria; cecal pH was significantly reduced, as were the activities of beta-glucuronidase and nitrate reductase
Lactulose
Lactulose is a synthetic disaccharide which was used originally as a laxative . Lactulose has also been shown to increase Lactobacilli and Bifidobacteria and significantly decrease bacteroides in mixed continuous fecal culture. In some clinical trials lactulose has been demonstrated to have potential for use as a prebiotic.Intake of lactulose, the number of Bifidobacteria increased significantly, whereas the numbers of Clostridium perfringens and Bacteroidaceae decreased.
Emerging and Novel Prebiotics
Xylo-oligosaccharides
Xylo-oligosaccharides (XOS) are chains of xylose molecules linked by β1–4 bonds which are produced enzymatically by hydrolysis of xylan from birch wood, oats, or corn cobs.
Resistant starch
Resistant starch is the term used for the fraction of starch that escapes digestion in the upper GI tract and that reaches the colon to be fermented by the colonic microbiota
Pectic oligosaccharides
Pectin is a complex galacturonic acid-rich polysaccharide which occurs naturally in the cell walls of higher plants and acts as a cement-like material for the cellulosic components of the plant cell wall. The major polysaccharides forming pectin include homogalacturonan (HGA), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II) which are believed to covalently link together .
Enzymatic or physical methods can be used to manufacture pectic oligosaccharides. Enzymatic hydrolysis of citrus and apple pectins in membrane reactors produces oligosaccharides of 3-4 kDa molecular weight . A nitric acid hydrolysis of citrus peel produces low molecular weight arabinose-based oligosaccharides . Both of these materials promote growth of Bifidobacteria.
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