Moderator: Francisco Guarner – Speakers: Karen Scott, Colin Hill


Karen Scott’s presentation was designed as a pedagogical introduction to the notion of prebiotics. She put forward the definition given in Gibson et al 2010:  “Prebiotics are a selectively fermented ingredient that results in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health”  Put into simple words, Prebiotics are “food” for the bacteria, normally living in the large intestine. Further to the definition, Karen Scott went on giving some elements of context about the gut microbiota.


She insisted on the fact that the gut microbiota was a complex and diverse bacterial community, composed of approximately 1000 different bacterial species colonizing each individual. Most of those bacteria are highly oxygen sensitive, but new molecular approaches allow us to analyze them without cultivation. In addition, gut microbiota has many functions: metabolism of dietary components, immune function, pathogenesis, and barrier function.  To fulfill those functions, she tried to define the composition of a gut microbiota, and more specifically suggested, based on observations, a characterization for a healthy composition. Gut microbes mostly fall into 4 phyla (in descending order of quantity): Bacteriodetes, Firmicutes, Proteobacteria and Actinobacteria. Citing Duncan and Flint (2009), she shows the proportion of total bacteria in healthy individuals that the 4 phyla can represent.

This illustrates how varied gut microbiota composition can be and that there are no standard composition for a healthy one.  Karen Scott then presented elements on the digestion of food in the gastro-intestinal tract. Most of the food is digested in the small intestine by human enzymes, whereas resistant starch, oligosaccharides are Prebiotics fermented by the gut bacteria procuring phytochemicals, metabolites… The microbial activity thus contributes to approximately 10% of host energy intake. By fermenting those foods, our gut microbiota produces Short Chain Fatty Acids (SCFA).

Of those, she puts forward 3 types known for their benefits:

• Butyrate, a major energy source for human colonocytes

• Proponiate important for the gluconeogenesis in the liver

• Acetate invovled in the fat loop to become fatty acids

Then Karen Scott explained the basic characteristics for a food to be qualified as a prebiotic and where prebiotics could be found.


A Prebiotic must resist digestion in the small intestine, be fermented by intestinal bacteria and selectively stimulate the growth/activity of ‘beneficial’ gut bacteria. Effect of prebiotics can either be direct or work in cross feeding effect. Prebiotics have reported health benefits for treatment of bowel cancer, IBD, diarrhoea, decrease cholesterol, Increase of calcium absorption, relieve constipation. They can naturally be found in garlic, leeks, onion and chicory. Also they are frequently added to cereals and yoghurts.  As a conclusion, Karen Scott gave her opinion on why we should use Prebiotics. They ultimately alter the SCFA production, which generally reduces colonic ph, inhibits growth of enteric pathogens, has a protective effect against colon cancer and decreases triglyceride, cholesterol synthesis.


Colin Hill on his side examined the topic of Probiotics. He started his presentation by highlighting the fact that probiotics were leaving the field of alternative medicine to reach that of evidence-based effects. One of his guideline statements throughout the rest of the presentation was that understanding Probiotics was a matter of definition, not of taxonomy. Genus, he continued, “does not confer a probiotic status”.  He reminded the commonly accepted definition for Probiotics: “Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host”.


On the contrary of our microbiota, Probiotics are allochthonous, ie they do not belong to our body, they are tourists that pass through our body rather quickly. Thus Probiotics are to be considered as an intervention. They temporarily overwhelm the autochthonous flora of the small intestine, before arriving in the large intestine as a small part of a numerous endegous microbiota. He observes a complex range of possible relationships between Probiotics, our Gut Microbiota and our Health. Probiotics may influence health directly, or by influencing the microbiota, or can modify dietary components.  Between January 2000 and December 2011, there have been more than 1000 publications on Probiotics.


The most commonly targeted conditions for probiotics are irritable bowel syndrome (IBS), antibiotic associated diarrhoea (AAD), Clostridium difficile associated diarrhoea (CDAD), pouchitis, necrotizing enterocolitis (NEC) and ulcerative colitis.  Colin Hill further cited O’Toole and Cooney 2008, where the authors identified 8 different mechanisms for probiotics to take action once inside the body. Those go as follows:

• Competition

• Bioconversions

• Production of vitamins

• Direct antagonism

• Competitive exclusion

• Barrier function

• Reduce inflammation

• Immune stimulation


Those mechanisms may be widespread (genus or species relatively unimportant), others may be frequent (genus or species important, strain relatively unimportant) and some may have rare effects (strain specific effects). As a conclusion Colin Hill highlighted the fact that understanding mechanisms of action will lead to better strain selection and greater confidence for practitioners and consumers.