28 Nov 2016
Nicola Ackerman
Job Title
Image © fserega / Fotolia
The gastrointestinal microbiome is the ecosystem consisting of microbiota (living bacteria, protozoa, viruses and fungi) living symbiotically in the host’s gastrointestinal tract.
Prebiotics are substances able to alter the gastrointestinal flora in a manner to benefit the microorganisms. Probiotics, however, are a live microbial feed supplement that benefit the host animal by improving the gastrointestinal microbial population.
Probiotics, generally used, are comprised of lactic acid bacteria, such as lactobacilli, streptococci and bifidobacteria. They have shown to be beneficial following acute gastroenteritis, or a course of antibiotics – especially in hindgut fermenters, such as rabbits and horses.
Probiotics, if administered, need to be given in large enough numbers and, potentially, on a daily basis. Daily oral supplementation of Enterococcus faecium (SF68, E 1705) has shown to significantly increase beneficial bacteria in the faeces and reduces quantities of potential pathogens, such as clostridia (Sparks and Jean-Philippe, 2012).
Yeasts have also been included in some probiotic preparations. Their role is to aid in improving the digestibility of fibre and other nutrients. Populations of yeasts do not seem to be maintained in the established gastrointestinal flora, thus, to maintain their effect, administration on a daily basis is required.
Two main mechanisms of action for probiotics exist, but in each of these the interactions are varied and complex (Sparks
and Jean-Philippe, 2012):
Modification of the bacterial flora of the intestinal tract:
Modulation of the immune system:
Prebiotics are specific nutrients that encourage the growth of beneficial bacterial population (for example, specific types of fibre).
Benefits the host will experience from this manipulation of gastrointestinal bacteria include:
VFAs benefit the animal by increasing available nutrients for gastrointestinal bacterial populations, which, in turn, aid in the quantity of nitrogenous waste materials from entering the bloodstream and causing azotaemia. This process is sometimes referred to as nitrogen traps in renal diets.
Manno-oligosaccharides are prebiotics that also aid in increasing the populations of certain microflora benefiting the animal. Its unique structure also attracts pathogens and bonds them to the manno-sugars, rather than attaching to the surface of the gut villi.
Glutamine is an amino acid commonly included in critical care nutrition diets, because of its immune-enhancing properties and ability to enhance wound healing. Glutamine is used in rapidly dividing cells, such as epithelial enterocytes and mucosal immune cells. Glutamine acts as a prebiotic by maintaining the overall health of the gut lining and, therefore, ensuring optimal nutrient absorption.
Fructo-oligosaccharides (FOS) act as a nutrient source for the beneficial bacteria of the gastrointestinal tract. FOS also increases gut transit time and draws water into the faeces –increasing bulk and softness.
Fibre plays an important role in the gastrointestinal system – acting as a prebiotic and influencing absorption and motility rates.
Manipulation of the fibre type and content within the diet can be used in the treatment and/or management of gastrointestinal disorders.
Numerous gastrointestinal disorders exist and with each individual reacting differently to different diets (and the fibre types and content) it is important to obtain a full nutritional/diet history from owners about their pets.
Other factors such as stress, water consumption, activity levels and genetics all play a part in many gastrointestinal disorders, hence a full detailed history incorporating these factors is required.
Fibre refers to a range of compounds classed as complex carbohydrates resistant to the action of digestive enzymes (Table 1). The primary function and benefit of adequate dietary fibre is to increase bulk and water in the intestinal contents, and it helps to promote and regulate normal bowel function and transit times. Fibres include cellulose, hemicellulose, pectin gums and resistant starches. Fibre is classified by its chemical structure, but also by its rate of fermentation by intestinal bacteria, digestibility and indigestible fractions, solubility in water, water-holding capacity and viscosity (Table 2; Gross et al, 2000).
| Table 1. Classification and digestion of complex carbohydrates (Gross et al, 2000) | |||
|---|---|---|---|
| Complex carbohydrate type | Function | Digestion site | Digestion products |
| Starch, glycogen | Storage polysaccharide in plants and animals | Small intestine (enzymatic) | Monosaccharides and disaccharides (glucose, maltose) |
| Hemicellulose, cellulose | Structural parts of plant cell walls | Large intestine (microbial fermentation) | Volatile fatty acids (acetate, propionate, butyrate) |
| Lignins, cutins, waxes | Associated cell wall substances | Not digested or fermented | Excreted in faeces |
| Gums, pectins, mucilages | Naturally occurring polysaccharides in plants | Large intestine (microbial fermentation) | Carbon dioxide, methane, hydrogen, volatile fatty acids |
| Table 2. Physiochemical and analytical properties of dietary fibre components (Gross et al, 2000) | ||||
|---|---|---|---|---|
| Carbohydrate and fibre fractions | Method | Fibre solubility | Total dietary fibre analysis | Crude fibre analysis |
| Fructans, galactans, mannans, mucilages | Rapidly fermentable | Soluble fibre | Total dietary fibre | |
| Pectin | Moderately fermentable | |||
| Hemicellulose | Insoluble fibre | |||
| Cellulose | Slowly fermentable | Crude fibre | ||
| Lignin | Not digested or fermented | |||
| Resistant starch | Moderately fermentable | |||
| Starch | Enzymatically digested | |||
| Monosaccharides and disaccharides | Absorbed | |||
Fibres rapidly fermented (such as pectins) by gastrointestinal bacteria produce more SCFAs and gases in a shorter period of time compared to fibre sources fermented more slowly, and this can lead to borborygmus and flatulence.
Pectins are commonly found in apples and citrus, and the most commonly used fibre sources in pet foods contain a mixture of pectins, hemicellulose and cellulose – this mix is classed as “moderately fermentable”. These fibre mixtures include rice bran, oat bran, wheat bran, soy fibres, soy hulls and beet pulp.
As the fermentation rate of the fibre used in the diet decreases, this will have the effect of increasing gastrointestinal transit time and faecal bulk. This can help to have the effect of increasing satiety in the animal and is often included in weight-loss diets.
These slowly fermentable fibres (for example, cellulose) are really effective in stool bulking agents because they retain their structure for longer and are, therefore, able to bind water in to the stool. This increase in faecal bulk/volume is advantageous for the treatment and prevention of irritable bowel syndrome and constipation.
The important end products of fibre fermentation are SCFAs, which include acetic, butyric and propionic acids, and are the preferred energy source of the coloncytes, which derive more than 70% of their energy requirement from luminally derived SCFAs (Bergman, 1990).
There is a rapid turnover of the epithelial cells within the gastrointestinal tract and, therefore, high energy needs. Dogs fed diets containing fermentable fibres have an increased colon weight, mucosal surface area and mucosal hypertrophy, compared to dogs fed non-fermentable fibre diets (Hallman et al, 1995).
These changes indicate an increased absorptive potential, which is of benefit to the animal as it will aid in preventing diarrhoea by enhancing the absorption of sodium.
This, in turn, maintains the normal intestinal electrolyte and fluid balance.
Other beneficial effects of SCFA production include promotion of the growth of indigenous microflora and inhibiting the proliferation of pathogenic microbes (Kerley and Sunvold, 1997) – acting as a prebiotic.
Hence, if probiotics are being administered to an animal, prebiotics are also required to promote their proliferation in the gastrointestinal system.
A vast amount of research is being undertaken in the role of probiotics and prebiotics. Some published work has looked at typing the biome, whereas others are looking at nutrients.
Reynolds and Satyaraj (2014) showed dogs fed diets supplemented with spirulina (two species of blue green algae – Arthrospira platensis and Arthrospira maxima) demonstrated enhanced immune status by showing a significantly higher vaccine response and higher levels of faecal IgA compared to a control group.
Supplementing diets with spirulina also resulted in significantly increased gut microflora stability in the test group. In conclusion, diets supplemented with spirulina significantly enhanced immune health and gut health in dogs, (Reynolds and Satyaraj, 2014).
Some other very interesting work has been looking into the possible links between the gut biome and skin disease. In a review of 13 randomised placebo-controlled trials (Betsi et al, 2008), probiotics, especially Lactobacillus rhamnosus GG, seemed to be effective in preventing human atopic dermatitis and reducing the severity of it in approximately half of the trials evaluated.
Other studies and meta-analyses of randomised controlled trials have suggested a role for probiotics in the prevention and treatment of human atopic dermatitis (Kalliomaki et al, 2001; Kim et al, 2014; Panduru et al, 2015).
Early exposure to probiotics, meanwhile, has been shown to have long-term clinical and immunological effects in a canine model of atopic dermatitis (Marsella et al, 2012).
