Looking at the various products that are marketed for gut health, it is understandable over the definitions, functions and modes of action of probiotics. There seems to be an overlap in their actions with prebiotics, even before the further confusion of synbiotics and postbiotics. Ultimately, probiotics play a role in adjusting or maintaining the natural microbial populations in the gastrointestinal tract; this may have implications on gut health, nutrient availability and digestion and the immunological status of the host animal. Although probiotics and prebiotics are markedly different products, and synbiotics & postbiotics are complementary, the results of their activities can be the same.
Probiotics are live microorganisms that, upon ingestion, modify the existing microbial population, gut physiology, and immunity. The earliest examples are “live” yoghurts; although not necessarily effective, they did excite the imagination!
Prebiotics are non-digestible feed components – generally but not exclusively oligosaccharides (short lengths of functional fibre) – that support certain microbial populations that effect a modification of populations and gut physiology. Early examples include chicory inulin.
Synbiotics are simply a selective blend of pro- and pre-biotics, the latter chosen to optimise the former.
Postbiotics are probiotics grown on a selective medium, possibly containing prebiotics. The resultant fermentate is then attenuated (microbes killed), and the entire mass is fed. Early examples would be spent brewer’s products.
However, now things are more sophisticated and for probiotics, there are several microbial species that are licensed to be used in the UK & Europe, more so across the world. They can be fed for upper gut colonisation – to help maintain a slightly more acidic environment by reducing starch to lactic acid or can be destined for hindgut function where they help maintain limited fibre fermentation, the resultant volatile fatty acids (VFA) having beneficial effects, and physically improving stool characteristics.
So, how do they work? Bearing in mind the vast numbers of bacteria in the gut and the (relatively) few added as probiotics it is more than just adding a new population. Probiotics can work in several ways.
Competitive exclusion
This term is used for the constant ingestion of probiotic bacteria overwhelming any potential pathogen bacteria that are also ingested. By denying them the ability to attach to the gut wall (by, in essence, getting there first!), pathogens are flushed through and removed.
Immune function
There are areas of the intestine called Peyer’s Patch, where the immune system is concentrated. Active cells, GALT & MALT, “recognise” probiotics and their fermentation end products, which stimulates a signalling cascade. This includes the maintenance of the mucosal barrier – a layer of mucus that lines the gut wall - a physical deterrent to pathogens, maintenance of tight junctions that help regulate the immunological release of IgA, and optimise access to the lymphatic system. This combination results in the suppression of the growth of pathogens as well as systemic and local anti-inflammatory effects.
Microbiota
The term microbiome is much used and describes 3 layers of function in the gut; the immune system (lymph system), the gut wall & mucosal barrier, and the microbial populations (microbiota). Some probiotic strains produce antimicrobial metabolites in large quantities. These include fatty acids, lactic acid, and acetic acid. Not only can these substances be antimicrobial, but they can also support acidophilic species such as lactobacilli; as such, they act in a prebiotic manner. Other antimicrobial substances, such as formic acids, free fatty acids, ammonia, hydrogen peroxide, diacetyl, bacteriolytic enzymes, bacteriocins, antibiotics, and several undefined substances, are produced in much smaller amounts but do modify the local populations.
Some microbial strains will utilise nutrients to produce short-chain fatty acids – such as lactic acid and VFA. Although the VFA butyric acid has been shown to help in the maintenance of tight spaces, nutrients such as lactic acid can be fermented by inherent populations, and this will improve numbers. Trial work has shown that cocktails of lactobacillus species can have positive effects on stool characteristics and reduce bacteria associated with diarrhoea.
Optimisation of the microbiota along its length can be influenced by selective choices in probiotic strains, directly as just mentioned, but also through their interaction with the immune system and impact on gut integrity and digestion.
Digestion
The microbiota can influence the digestive environment, and probiotics can influence the microbiota. The digestion efficiency can be improved depending on the fermentative base – and prebiotics have been shown to have a critical impact on probiotic influence. At the same time, the VFA butyric, which can influence tight junction & immune function and the microbiota, also energises the cells of the gut lining, directly affecting absorption. In the hindgut, this further protects gut health as tight junction control helps reduce the absorption of large molecule endotoxins and optimises faecal consistency.
There are various commercial preparations available. Some, based on single species – such as E. faecium, L. acidophilus – and some on combinations, may all benefit the health and integrity of the microbiome. They are, though, in the main, offered alongside prebiotics. This enables a nutrient base for hindgut function (foregut nutrients are ideal for some stains, such as L. acidophilus). Research has shown that a probiotic/prebiotic combination is more effective than probiotics alone. Again, there are a range of prebiotic types – fructo-, manno-, pecto- oligosaccharides, etc. – and these can influence individual probiotic strains. In addition, postbiotics supply those nutrient, digestive & immunostimulant cues without the live micro-organisms. This is important as the yeast probiotic is currently not licensed in the UK for dogs/cats (although it is in Europe), but it is an effective product. Most of its benefits are included in supplying it as a postbiotic.
Combinations of pro-, pre-, and postbiotics can create an effective and efficient gut support mechanism.