Title: Gut Microflora and Diet

Key words: microbial ecosystem, fermentation, dietary modulation, probiotics, prebiotics, synbiotics, beneficial flora

Date: Jun 1999

Category: 8. The Gut

Type: Article

Author: Dr van Rhijn

Gut Microflora and Diet


The human large intestine is a complex microbial ecosystem and the most heavily colonised region of the digestive tract (1012 bacteria /gram content). Gut fermentation is a complicated process, in which the wide range of bacterial species is responsible for multiple nutritional patterns1. These species act interdependently to bring carbohydrate metabolism to full completion. Colonisation and growth of bacteria in the intestines are determined by numerous factors, such as substrate availability (nutrient competition & composition of the diet2), gut transit time and pH (autoinhibition of invaders) and redox potential (inhibition by deconjugated bile acids).

Why Modulate Microflora?

Colonic microbiota play a major role in host health3 and there is some interest in manipulating a shift in colonisation from potentially harmful and pathogenic bacteria (for example clostridium and pseudomonas) to more beneficial and health promoting groups (such as lactobacilli and bifidobacteria). Pathogenic effects include diarrhoea, infections, liver damage, carcinogenesis, chronic degenerative disease and intestinal putrefaction. On the other hand, promoting effects may exert an antibacterial effect (acetic & lactic acid production and toxin suppression)4, boost resistance to infections (immunomodulators), modulate metabolism by lowering blood ammonia and cholesterol levels, improve digestion, and thus aid absorption of essential nutrients and synthesis of vitamins.

Dietary Modulation

Microbiota can be influenced by diet in the following ways:

Successful probiotics are viable, live microbial food supplements, which bypass digestion and become metabolically active in the colon5, beneficially affecting the host animal by improving its intestinal microbial balance6. They should be non-pathogenic, non-toxic and have good taste and storage viability. They are obtained from fermented milk products7. The classes involved are:

Thermophillic (Lactobaccillus [Buttermilk]),

Bio-cultures (Bifidobacterium [Bio-yoghurt]),

Mesophilic (Streptococcus [Cottage cheese])

and Lactic acid bacteria [Kefir].

Probiotics also alleviate symptoms of lactose malabsorption (lactase activity) and may help suppress cancer (apoptosis). Their effect may, however, be transient8.

These are naturally occurring, non-digestible food ingredients, that beneficially affect their host by selectively stimulating the growth and/or activity of one or a limited number of bacteria already resident in the colon and thus improve host health9. Examples are xylose, mannose, fructo-oligosaccharides (FOS), polymeres such as oligofructose10 and inulin11, 12 and galacto-oligosaccharides. FOS occur naturally in vegetables such as onion, asparagus, banana, artichokes, chicory and commercially as Raftilose. They are especially fermented by bifidobacteria and 15g/day in the diet results in colonisation of these beneficial bacteria13.

Implies mixtures of pro- and prebiotics, which beneficially affect the host by improving the survival and implantation of live microbial dietary supplements in the gastrointestinal tract. Combinations such as FOS + bifidobacteria or lactitol + lactobacilli. Synbiotics must determine prebiotic fermentation, define probiotics, and be digestion resistant, stable, tasty and efficacious.


Increased levels of certain bacteria have been implicated as causative agents in many colonic and systemic disorders14. As probiotic and prebiotic therapy appears to be effective in these conditions15 they, especially synbiotics, have potential non-invasive, pharmaceutical applications for treatment and in control of food poisoning organisms16. Further research on development is required to utilise their full potential as dietary products to stimulate beneficial flora.


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