The Food and Agriculture Organization of the WHO provides the most widely accepted definition of a probiotic as 'a live organism that, when ingested in adequate amounts, exerts a health benefit to the host'. The most commonly used probiotics are lactobacilli, bifidobacteria and nonpathogenic yeasts such as Saccharomyces boulardii. Products that are labelled 'probiotic' are now widely available; however, few fulfil this definition. From a microbiological perspective, some may not contain sufficient live organisms following commercial or domestic storage or have not been adequately tested to ensure they will survive transit through the gastrointestinal tract. From an application perspective, some may not confer a claimed health benefit either because they have not undergone efficacy testing in humans or because what evidence is available is inadequate or negative.
In the future, the definition of a probiotic may require modification, as there is experimental evidence that dead bacteria, bacterial components and substances secreted by bacteria (e.g. bacteriocins, conjugated linoleic acid) have physiologically relevant effects. The more inclusive term 'pharmabiotic' has also been proposed to encompass entities that exert these potentially important effects.
Many probiotics have been used for decades, and yet, definitive data on safety are limited. In the largest ever systematic review of probiotic safety, the Agency for Healthcare Research and Quality concluded that 'the available evidence in randomised controlled trials does not indicate an increased risk; however … despite the substantial number of publications, the current literature is not well equipped to answer questions on the safety of probiotic interventions with confidence'. Most would agree that probiotics should be used with caution in certain patient groups. There have been a small number of notable complications of probiotics in the acute clinical setting, the first reporting septicaemia in infants with short bowel syndrome and the second reporting increased mortality among patients with severe acute pancreatitis administered a novel probiotic mix as a result of intestinal ischaemia of unclear cause.
The manner in which the production, advertising and sale of probiotics is regulated has varied considerably between countries. There has been a regrettable trend for some products to claim benefits for their strain(s) on the basis of evidence from other strain(s). Probiotic effects are strain specific and cannot be extrapolated from one strain to another, no matter how closely related. Regulation in this area is changing, especially where health claims are involved, as indicated by the recent pronouncements of the European Food Safety Authority, which now demands robust data to support health claims.
A major advance in probiotic research has been the greater understanding for their mechanisms of action. The gastrointestinal microbiota and some probiotics have considerable metabolic activity, including the fermentation of nondigested carbohydrates and their conversion into short-chain fatty acids, the deconjugation of bile salts and vitamin synthesis. A number of probiotics modify the inflammatory response to some enteropathogens. For example, a specific strain of Bifidobacterium infantis 35624 has been shown to prevent nuclear factor-kappa-B and interleukin (IL)-8 activation and also inhibit the secretion of chemokine ligand 20 in response to Salmonella typhimurium, Clostridium difficile and Mycobacterium paratuberculosis.
Some probiotics have been shown to produce chemicals (e.g. neurotransmitters, neuromodulators) that can modify gastrointestinal functions such as motility or sensation and some have been shown to enhance mucosal barrier function and modulate inflammation. These mechanisms suggest potential roles for probiotics in the management of IBS and IBD. Within the preceding year, numerous mechanistic studies, randomized controlled trials (RCTs), systematic reviews and meta-analyses relating to this area have been published. The aim of this article is to review the recent evidence for the mechanisms and effectiveness of probiotics in the management of IBS and IBD.
Curr Opin Gastroenterol. 2013;29(2):184-189. © 2013 Lippincott Williams & Wilkins