Gastrointestinal tract 6: the effects of gut microbiota on human health

The combined gut microbiota (the community of micro-organisms in the gastrointestinal tract), exerts powerful and diverse effects on physiology. It plays a role in synthesising useful biologically active molecules, modulating immune responses, behaviour and mood. Dysbiosis, where the balance of the microbiota is disrupted, is associated with increased risk of certain diseases, conditions and autoimmune reactions. This article, the last in a six-part series on the gastrointestinal tract, examines the gut microbiota and its role in the body. Citation Knight, J et al (2019) Gastrointestinal tract 6: the effects of gut microbiota on human health. Nursing Times [online]; 115: 11, 46-50. In this article. ● Gut microbiota and its role in the gastrointestinal tract ● The diverse effects of the gut microbiota on human physiology ● How imbalances could increase risk of certain diseases and conditions Clinical Practice Systems of life GI tract Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use 47 Nursing Times [online] November 2019 / Vol 115 Issue 11 www.nursingtimes. net 2kg of gut microbial biomass found in this location. Increased microbial diversity in the colon reflects significantly reduced enzyme activity in this region, which is primarily dedicated to water and salt reabsorption (see part 5). The pH of the colon is also more favourable, being slightly acidic to neutral (typical pH5.5-7.0). Effects on human physiology Research into how the microbiota affects physiological processes is still in its infancy. One of the major problems in this field of study is the inability to culture many of the key bacterial species found within the gut. Culturing is essential to allow investigation of the biochemistry of gut microbes and identify the microbial metabolites that can modulate human physiology. Despite this problem, useful information on the role played by the gut microbiota in modulating human physiological processes has recently been established. Role in digestion The microbial communities of the large intestine are predominantly reliant on partially digested food arriving from the small intestine for nutrition and survival. The gut microbiota itself plays an active role in digestion of the carbohydrate, fat and protein components of food. Bacteria residing in the large intestine play a particularly important role in digestion of dietary fibre to yield short-chain fatty acids (SCFA) such as acetate, propionate, and butyrate. Propionate is thought to play an important role as a satiety molecule, with the ability to contribute to the switching off of hunger, while butyrate promotes programmed cell death (apoptosis) in malignant epithelial cells lining the large intestine, thereby reducing the risk of bowel cancer. The digestion and fermentation of dietary fibre by the gut microbiota yields large quantities of gasses such as odourless methane, carbon dioxide and hydrogen, together with smaller amounts of pungent odoriferous gasses such as hydrogen sulphide (Rowland et al, 2018). Vitamin biosynthesis Human cells and tissues are unable to synthesise directly many of the essential vitamins required for health and survival, and so these must be acquired through a combination of dietary intake and biosynthesis by the GI tract microbiota. A healthy, wellbalanced microbial community rich in Bifidobacteria, Lactobacilli and E coli is able to synthesise many of the water-soluble vitamins, including key B vitamins – such bacterial colonisation. Unsurprisingly, those born via natural vaginal delivery have an early gut microbiota that is similar to the vagina, with groups such as Lactobacillus dominating. This contrasts with babies born via caesarean section, who have early gut microbiota similar to that found on the mother’s skin, with groups such as Corynebacterium and Staphylococcus species present in high numbers (Dominguez-Bello et al, 2011). The diversity of bacterial species generally increases with age, as different species are acquired from environmental contact, particularly from eating different foods and through contact with other people and animals. It is thought the acquisition of a mature microbiota resembling that of an adult is attained in the first three years of life, with country of residence also affecting the microbiotic profile (Yatsunenko et al, 2012). Differences along the gut Due to vastly different conditions, particularly in terms of pH, the populations of bacteria and other micro-organisms vary markedly in the different gut regions (Fig 2, p48). Unsurprisingly, the strongly acidic conditions of the stomach limit microbial colonisation and so relatively few species of bacteria are able to survive there. Notable exceptions are Lactobacillus species and Helicobacter pylori, which is a key bacterium linked to the formation of gastric ulcers (see part 2). Similarly, active proteases enzymes, such as trypsin, chymotrypisin and intestinal peptidases, limit the growth of bacteria in the small intestine, which is dominated by Lactobacillus and Streptococcal species. As can be seen in Fig 2, the colon plays host to the most diverse communities of bacteria, with the vast bulk of the estimated recognised that not only does the microbiota play a role in synthesising useful biologically active molecules, it is intimately involved in modulating immune responses and also influences behaviour and mood. Of particular interest is the observation that dysbiosis, where the normal balance of species within the microbiota is disrupted, is associated with increased risk of certain diseases and medical conditions, including Parkinson’s disease, autism, obesity, diabetes and certain autoimmune reactions. Microbial species forming the gut microbiota A major challenge in trying to identify the microbes that live within the human GI tract is that many cannot be grown using the standard microbiological culturing techniques. However, the advent of modern genome sequencing has allowed rapid identification of micro-organisms without the need for culturing. The bacterial species of the gut have been the most heavily researched, while knowledge of the viral and fungal micro-organisms of the gut microbiota is currently sparse. The low oxygen levels present within the internal environment of the GI tract favours the growth of strict anaerobic species of bacteria, which greatly outnumber facultative anaerobes (bacteria that can switch between aerobic and anaerobic metabolism depending on oxygen concentration) and the aerobic species (bacteria that require the presence of oxygen) (Sekirov et al, 2010). Two major groups of bacteria dominate the GI tract (Fig 1) with the Firmicutes (for example, Lactobacillus and Streptococcus species) comprising around 65% of the total and the Bacteroidetes (for example, Bacteroides intestinalis) around 30%. The remaining 5% are composed of primitive bacterial groups, including the proteobacteria (for example, E coli) and the actinobacteria such as Bifidobacteria species (Yang et al, 2009). It is estimated that all individuals have between 500 and 1,000 species of bacteria colonising their gut. Population studies indicate much variation between individuals, with each person having a unique profile of microbial species. It is suggested up to 35,000 species form the collective human gut bacterial microbiota, with new species continually being discovered, including many previously unknown to science (Barras, 2019). Initial colonisation after birth Babies emerge from the usually sterile environment of the uterus with minimal Clinical Practice Systems of life Fig 1. Human gut microbiota