Probiotics and Prebiotics: effects on the gut microbiota

Published on 11 November 2010 in Food, health and wellbeing

Gut bacteria digesting cellulose


There has been a significant increase in the number of food products containing either probiotics or prebiotics reaching the supermarket shelf in recent years. Equally the myriad of health claims associated with these products is confusing to say the least: some 'improve symptoms of constipation', others 'prevent the onset of diarrhoea' while still more are claimed to 'recreate the natural balance of your gut bacteria'. Whilst there is sound scientific evidence supporting some of these claims, there are other products which appear to 'piggyback' on these genuine claims, without any substantiated evidence of an effect associated with that specific product.

The human body is host to an enormous number of bacteria, the vast majority of which colonise the gastrointestinal tract (GIT), from mouth to anus. In fact there are more bacterial cells than human cells in the human body! The highest bacterial density is found in the large intestine, where the pH, anaerobic conditions and availability of nutrients create a perfect environment for bacterial growth. There are 1011 – 1012 intestinal bacterial cells per gram of contents, belonging to more than 500 distinct species. These bacteria rely on dietary substrates escaping digestion in the upper intestinal tract for growth and survival, producing many metabolites which can be beneficial to the host. The predominant end-products are the short chain fatty acids, of which butyrate is a key energy source for the cells lining the intestine, while propionate and acetate are transported round the body where they perform other important functions.

Key Points

Probiotics are (by definition) – “live microorganisms which when administered in adequate amount confer a health benefit on the host” (FAO, 2001).

Most probiotic bacteria are strains of Bifidobacteria and Lactobacilli, which are present in low numbers in the normal gut microbiota, and which are generally alleged to be associated with health. Since different bacterial strains, and above all species, have different attributes and properties, it is impossible to generalise about health claims associated with probiotics. Each individual strain has to be tested extensively, and any claim has to be product-specific.

Dietary prebiotics on the other hand are “selectively fermented food ingredients that result in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health’’.

Most prebiotics are based on inulin and shorter fructo-oligosaccharides. Prebiotics can be supplemented into foodstuffs (often cereals and breads), or may be found naturally within plant foodstuffs (artichokes, leeks and onions are rich sources of the fructo-oligosaccharide type of prebiotic). They are resistant to stomach acids, and indigestible by human enzymes, but are degraded by specific bacterial species in the colon, including Bifidobacteria and Lactobacilli. Since prebiotics stimulate the growth and activity of resident intestinal bacteria, they do not suffer from the viability problems that can be associated with probiotics.

Research Undertaken

The development of molecular tools to enumerate specific bacterial species has facilitated an increase in our knowledge of the composition of the intestinal microbiota. Scientists at the Rowett Institute of Nutrition and Health (RINH) have contributed to improved knowledge of the roles of specific groups of abundant bacteria in the gastrointestinal tract (GIT), and have built up an important culture collection of representative bacteria from all the important bacterial groups resident in the human GIT.
Research using the bacterial isolates, individually or in combinations, enabled us to determine the key roles of some these bacteria within the GIT:

  • Some of the most abundant bacteria found in the GIT are able to utilise inulin and fructo-oligosaccharides for growth, to the same extent as Bifidobacteria, producing butyrate as a metabolic product. The prevalence of these same bacteria can be stimulated by the addition of prebiotics both under laboratory conditions, and in human studies.
  • Genetic analysis has enabled us to identify the most important genes required for growth and uptake of these substrates. One bacterium, able to use long-chain substrates for growth, was apparently able to internalise substrate particles by expanding the capacity of a known transport mechanism.
  • The response of anyone to probiotic or prebiotic supplementation is individual-specific, and strongly dependent on the inter-individual variations in the existing resident microbiota.
  • This research has identified additional resident bacterial species that are promoted using prebiotics, with considerable benefits for human health. Additionally some of the bacterial species which have been shown to be numerous in the healthy GIT, could potentially be developed as novel probiotic bacteria.

Policy Implications

All food products containing prebiotics or probiotics have to be approved by the European Food Safety Authority (EFSA) prior to reaching the market place. Part of this approval process requires valid scientific evidence in support of any claims that products “improve health”. Research carried out at RINH provides clear, objective and unbiased guidance on the health impact of these products in different groups of consumers. Such basic research is essential for informed assessment of the validity of claims by EFSA.


Dr Karen Scott


Food, health and wellbeing

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