The Role of Prebiotics in Human Gut Microbiology

Abstract

The human large intestine is an intensely colonised area. This is because the colon contains a vast array of microorganisms, thought to represent the majority of all cells in the body. The nutritional, biological and clinical importance of bacteria resident in the gut is becoming increasingly recognised. Although it is known that many disease states involve bacterial metabolism, the human gut microflora may also be considered as relevant to an improvement in host health and welfare. Gut bacteria carry out a multidisciplinary process known as fermentation, where dietary and indigenously produced residues are metabolised to a variety of different end products. The numerically predominant (culturable) anaerobes in the gut are Gram-negative rods belonging to the genus Bacteroides. Other groups which have been identified as significant include bifidobacteria, clostridia, eubacteria, lactobacilli, Gram-positive cocci, coliforms, methanogens and dissimilatory sulphate-reducing bacteria. It is thought that several hundred different bacterial species are present in the human large intestine. The principal substrates for colonic bacterial growth are dietary carbohydrates which have escaped digestion in the upper gastrointestinal tract. These may be derived from either the diet or by endogenous secretions and include both carbohydrates and proteins. Whilst the products of gut proteolysis may be generally thought of as toxic towards host health, those of saccharolytic digestion may be considered to be benign and in some cases can contribute positively. In humans, there are positive aspects to the gut fermentation which may improve certain aspects of host health. The microflora contains certain bacteria that can be perceived as health promoting, as well as pathogenic. For instance, bifidobacteria and lactobacilli may help to improve resistance to gut infections by inhibiting the growth of harmful bacteria, reduce blood lipid levels, improve the immune response and be involved in protection against gut cancers. Whilst the definitive health outcomes are not clearly defined, there is currently much interest in increasing numbers and activities of these bacteria in the large gut, preferably at the expense of more harmful species. The manner in which this can be achieved is through dietary supplementation. The use of probiotics has been widely supported. In this case, foodstuffs such as fermented milk products containing viable cultures perceived as beneficial (e.g. lactobacilli, bifidobacteria) are used to proliferate populations in the colon. Probiotics are defined as live microbial feed supplements which beneficially affect the host animal by improving its intestinal microbial balance. To be effective, probiotics must be capable of being prepared in a viable manner and on large scale (e.g, for industrial purposes), whilst during use and under storage the probiotic should remain viable and stable, be able to survive in the intestinal ecosystem and the host animal should gain beneficially from harbouring the probiotic. Some of these requirements may be difficult to attain. An alternative, or additional, approach is the prebiotic concept. A prebiotic is a non digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, that can improve the host health. Thus, the prebiotic approach advocates the administration of non viable entities. Dietary carbohydrates are candidate prebiotics but most promise has been realised with oligosaccharides. In particular the ingestion of fructooligosaccharides has been shown to stimulate bifidobacteria in the lower gut. As prebiotics exploit nonviable food ingredients, their applicability in diets is wide ranging.