A BENEO-Orafti Newsletter - Spring 2008 - Nr. 18:
Prebiotics and healthy ageing

Interview

How do gut microbiota help to prevent disease?

Human research shows that a healthy equilibrium of gut microbiota helps to repel exogenous pathogens. We are not sure of the exact mechanisms but we think that the microbiota are antagonistic to pathogens and beneficially modulate the immune response via systems such as ‘cross talk’. This is where commensal bacteria and the gut-associated lymphoid tissue (GALT) interact using a complex array of chemical signals. Toll-like receptors on our gut cells are able to recognise signals given by the microbiota and will uprate the immune system in response. Another way that the microbiota might act is via competition for food. As commensal bacteria are well adapted to the host environment, they are able to compete more effectively for food than pathogens. Microbiota may also help reduce the potentially genotoxic effect of certain compounds present in food residues, e.g. nitrosamines from meat, by converting these into non-toxic compounds.

How does the balance in gut microbiota change as people age?

As we age, we become exposed to a greater diversity of bacteria and the balance between different phyla/genera of bacteria also changes. One example is the ratio of Firmicutes to Bacteroidetes. The Firmicutes phyla describes bacteria that mostly have a gram-positive cell wall structure and includes genera such as Bacillales (e.g. Staphylococcus), Lactobacillales (e.g. Lactobacillus) and Clostridia (e.g. Clostridium leptum and Clostridium coccoides). In contrast, the Bacteroidetes (e.g. Bacteroides) are gram-negative anaerobic genera. Both are commensal but the ratio between them changes from 90:10 Firmicutes:Bacteroidetes in adults to 50:50 ratio in infants and in the elderly. Reasons for these differences include changes in our secretions as we age. Secretions, i.e. mucus and cell debris, make up half of faecal material (the other half is food residue). If the secretions and/or diet change, this will have a major influence on the types of microflora able to thrive in the gut environment. However, there is still much to learn as we have only been able to cultivate around 90% of the microbiota in infants and less than 20% in the elderly. Gaps in knowledge of the human gut microbiota have led to assumptions that levels of bifidobacteria reduce and pathogens increase as we age, which may not be fully true. Changes in methodologies away from cultivation and towards metagenomics will help us identify more of the microflora in the human gut and determine the changes that occur as we age.

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