THE gastrointestinal (GI) tract contains a complex ecosystem of micro-organisms known as the microflora (which includes bacteria, archaea, fungi, protozoa and viruses).
These micro-organisms can be grouped into commensals, parasites or mutualists. It is estimated that there are between 1012 to 1014 micro-organisms within the GI tract of mammals. This is about 10 times more than the number of cells composing the host body.
The microbial ecosystem within individual dogs and cats is unique. It has been shown that there is only a minor overlap of bacterial species between individual animals. Instead, most mammals share similar bacterial phyla and genera.1 This article summarises the role of the microflora in health and disease, with a particular focus on the use of probiotics in veterinary medicine.
The role of the microflora in healthy animals
Gut microbes play a vital role in host health: they form a barrier against pathogen colonisation, aid digestion and also stimulate the normal development of the host’s immune system.
The presence of bacteria within the GI tract early in an animal’s life is vital in order to establish an oral tolerance to commensal bacteria and normal food antigens. Without such a tolerance, there can be an inappropriate immune response, which can lead to the development of diseases such as inflammatory bowel disease (IBD).
Additionally, it is now known that the commensal microflora contributes significantly to the development and maintenance of the GI tract structure. For instance, germ-free dogs have been shown to have thinner villi and a reduced mucosal surface area.
The resident microflora help to protect the body from invading pathogens via the process of “colonisation resistance”. Several mechanisms for this have been proposed including the competition for nutrients, oxygen and mucosal adhesion sites, as well as alterations to the local GI tract environment so that non-resident bacterial species fail to thrive (e.g. alterations in the pH and antimicrobial secretions).
The microflora in disease
Specific pathogens may lead to GI disease (e.g. Salmonella spp., Campylobacter jejuni, Escherichia coli and Clostridium perfringens). Clinical signs that can be seen when there is an enteric bacterial infection include diarrhoea (with or without blood and mucus), vomiting, inappetence, abdominal pain and pyrexia.
It is now known that an altered microflora plays a role in various GI disorders. For example, studies have shown that there is a reduced species richness seen in dogs and cats with IBD – often there are depleted numbers of Bacteroidetes. Dogs and cats with idiopathic small intestinal IBD also appear to have a higher proportion of Enterobacteriaceae, such as E. coli, compared with controls.
Other factors like dietary change, antibiotic treatment and stress (e.g. rehoming and travel) may lead to a disruption in the microflora (known as a state of dysbiosis) which in turn may predispose to GI disease.
Probiotics have been defined as “live micro-organisms which when administered in adequate amounts confer a health benefit on the host” by the joint Food and Agriculture Organisation (FAO) and the World Health Organisation (WHO). Traditionally, probiotics have been consumed in the form of fermented foods (e.g. yoghurt, sauerkraut and cheese); however, probiotics are now available in more specialised preparations including pastes and capsules.
Probiotics can be used to manipulate the microflora to a more favourable balance. This is especially useful when there is some form of dysbiosis within the GI tract. By providing an influx of probiotics, a normalisation of the microflora may be achieved.
The precise mechanism of probiotic action is still not fully understood; however, there are several ways in which probiotics are thought to work, and a summary of these functions is shown in Table 1.
One way in which probiotics can exert a beneficial effect is through their competition with pathogenic bacteria for adhesion sites and nutrients within the GI tract. This process is termed “competitive exclusion”.
Certain probiotics (e.g. lactobacilli and enterococci) are also able to produce organic acids, such as lactic acid, which lower the pH of the probiotic’s immediate environment. This has the effect of inhibiting the growth of pathogenic bacteria such as E. coli and Salmonella species.
Studies have shown that the probiotic Enterococcus faecium (NCIMB 10415) is able to decrease levels of C. perfringens within the faeces of dogs and cats.
It has also been shown that probiotics can lead to an increase in mucus levels within the GI tract, as well as being able to exert a protective effect upon the epithelial barrier. Probiotics that can help to regulate intestinal epithelial function may have a role in chronic enteropathies such as IBD.
Recently, attention has been paid to the abilities of probiotics to enhance and regulate the immune system. Probiotics have been shown to induce the development of T regulatory cells (Tregs). T-cells are essential for the maintenance of a normal immune balance in the GI tract and other organ systems via interactions with dendritic cells.
Probiotics have also been shown to up-regulate IL-10 (a regulatory cytokine) and to a lesser extent TGF-ß.9 IL-10 and TGF-ß, which are produced by subsets of regulatory Tlymphocytes, help to down-regulate the expression of proinflammatory cytokines in other
immune cells and to modulate them. Probiotics may therefore be of benefit in IBD cases, where a dysregulation of the immune system can be seen.
Probiotics available for use in dogs and cats
For admission onto the European common market, probiotics intended for animals have to be assessed according to the Feed Additive Regulation (EC) No 1831/2003. At the time of writing (May 2011) the only probiotics registered for use in the European Union for cats and/or dogs are two forms of E. faecium (NCIMB 10415) and the strain Lactobacillus acidophilus (DSM13241).
Within the UK, the main probiotic products that are available include ProKolin+ (Protexin Veterinary), Canikur Pro (Boehringer Ingelheim) and FortiFlora (Nestlé Purina). Both ProKolin+ and Canikur Pro contain the probiotic E. faecium (NCIMB 10415) E1707 and Fortiflora contains the probiotic E. faecium (NCIMB 10415) E1705.
It is now well-known that the microflora play a vital role towards the health of the GI tract. Alterations of the microflora can lead to digestive upsets, and there is emerging evidence that probiotics may be a useful tool in the maintenance of normal digestive function.
- Ley, R. E., Hamady, M. and Lozupone, C. et al. (2008) Evolution of mammals and their gut microbes. Science 320: 1,647-1,651.
- Bauer, E., Williams, B. A. and Smidt, H. et al. (2006) Influence of the gastrointestinal microbiota on development of the immune system in young animals. Curr Issues Intest Microbiol 7: 35-51.
- Thompson, G. R. and Trexler, P. C. (1971) Gastrointestinal structure and function in germ-free or gnotobiotic animals. Gut 12: 230-235.