What are Probiotics?

A "probiotic", by the generally accepted definition, is a "live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance". Although referring to the supplementation of animal feeds for farm animals, the definition is easily applied to the human situation. The major consumption of probiotics by humans is in the form of dairy-based foods containing intestinal species of lactobacilli and bifidobacteria. It is implicit in the definition that consumption of the probiotic affects the composition of the intestinal microflora. This effect of the probiotic on the intestinal ecosystem, it is proposed, impacts in some beneficial way on the consumer. A number of potential benefits arising from changes to the intestinal milieu through the agency of probiotics have been proposed, including:

· increased resistance to infectious diseases, particularly of the intestine· decreased duration of diarrhoea· reduction in blood pressure· reduction in serum cholesterol concentration· reduction in allergy· stimulation of phagocytosis by peripheral blood leucocytes· modulation of cytokine gene expression· adjuvant effect· regression of tumours· reduction in carcinogen or co-carcinogen production

Perhaps surprisingly, despite this impressive list of therapeutic and prophylactic attributes, probiotics are not commonly part of the medical practitioner's armamentarium of prescription drugs. Instead, probiotics are available from retail outlets, usually supermarkets, grocery and health food stores. The probiotics are available to the consumer as powders or tablets, but most commonly as milk-based products. The growth in the production of probiotics by the dairy industry means that it is now increasingly difficult to purchase yogurts that do not contain "probiotic" bacteria such as Lactobacillus acidophilus. They sell well, but it is very doubtful that probiotic yogurts are purchased entirely for health reasons. Consumers may be purchasing probiotics with the vague idea that it is "good for them": that consumption of the product will contribute to their well-being. Since even some scientists treat the "probiotics" concept with scepticism, how can the average consumer hope to comprehend the significance of "acidophilus" or "bifidus" in any but the simplest of contexts? Many others purchase probiotic dairy products because they prefer the organoleptic and rheologic characteristics of these products in comparison to "regular" yogurts. Whatever the reason, there are consumers who hold a firm belief that their health is improved by regular consumption of a probiotic. It becomes almost an article of faith. Others are suspicious of the probiotic industry, and question the validity of the claims made in relation to health benefits. And, it must be admitted, they have some basis for these doubts: the marketing strategy for probiotics relates to health benefits and therefore to medical science. Yet even the most ardent advocates of the probiotic concept must admit that the "science" associated with probiotics over many decades has been remarkably weak. Why else has the medical profession, and much of the scientific community, remained aloof from the results of probiotic research?

Despite doubts in scientific circles as to its validity, the probiotics industry is flourishing, and interest in establishing scientific credibility has attained importance for many companies and scientists. European Union programmes are to the fore in this work. It is an unusual and difficult situation for scientists, however, because probiotic products have been in existence for decades, yet they must now search for experimental evidence to support long-held beliefs associated with products already in the retail market. The probiotics industry is burdened with myth (anecdotal studies) and a reliance on in vitro experimentation. Much effort has been devoted to screening bacterial isolates for properties deemed appropriate for a "probiotic" strain, mostly characteristics that might enable the microbes to at least survive passage through the digestive tract. There must be millions of such strains to choose from, because the intestinal milieu of humans is already the home to bacteria with these properties. Probably the biggest obstacle to allaying scepticism is that the probiotic concept is based on a very poor understanding of the intestinal microflora. The concept concentrates essentially on two groups of bacteria, lactobacilli and bifidobacteria, while practically ignoring the vast array of other species that inhabit the intestinal tract of humans. It is the impact of probiotics on the composition of the intestinal microflora, nevertheless, that forms the basis for the probiotic concept. What can currently be said about the impact of probiotic bacteria on microbial balance at the moment? "They go in at one end of the digestive tract and come out the other, and hopefully something good happens along the way" is probably not too harsh a statement. Can anyone define the "microbial balance" to which one aspires in the definition of a probiotic?

While there is a current trend to conduct "clinical trials" to prove the efficacy of exisiting probiotics, it may be better to return to a consideration of fundamental principles of microbial ecology, human physiology and immunology before embarking on these very costly exercises. What are the "beneficial effects" that one wants to produce by using probiotics? Do we know how to recognise the beneficial effects mentioned in the definition of a probiotic? Better, perhaps, to allocate funds to the utilisation of molecular biological tools in the analysis of the complicated systems which must be investigated. With these tools, a better understanding of the interactions between members of the intestinal microflora and between the microflora and the human host could be established. The intestinal bacteria could then be used as mediators to modulate phenomena that are of significance to human health. The development of modern pharmaceutical drugs is based on fundmental knowledge of processes occurring within the human body. Mechanisms of drug action are known, and explanations of efficacy can be publicised. To attain scientific validity, probiotics must be derived by the application of logic.