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Introduction
Certain species of the resident bacteria are known to provide benefits to the host. These are the lactic acid producing bacteria (LAB), which include the Lactobacilli, Bifidobacteria, members of the Streptococci, and several other species. The amount and composition of substances reaching the large intestine can be readily modified by diet and, in terms of bacterial substrates, it is the fermentable substrate such as oligosaccharides, starches and non starch polysaccharides (NSP) which are most important. Prebiotics have been defined as “nondigestible food ingredients that selectively stimulate growth and (or) activity of potentially health-enhancing intestinal bacteria”. The nondigestible oligosaccharides are characterised by being resistant to hydrolysis by the enzymes secreted by vertebrates, but can be fermented by bacteria in the large intestine (Roberfroid, van Loo and Gibson, 1998). Oligofructose and inulin are the two most commonly researched and commercially available oligosaccharides.
Inulin and oligofructose
Some research has reported improved growth performance and feed efficiency (FCR) (Pierce et al., 2005) following inulin inclusion. Pierce et al. (2005b) found that the inclusion of inulin in piglet diets resulted in improved FCR and a numerical increase in daily gain in the first week post weaning. The improved FCR with inulin may be due to (i) improved gastrointestinal health, (ii) increased production of short-chain fatty acids from the fermentation of inulin and (iii) increased food intake.
Pierce et al. (2005b) compared the effects of inulin with two levels of lactose (175 and 295 g lactose/kg) on piglet performance and diet digestibility post weaning. Supplementing high lactose diets with inulin gave no benefit in terms of pig performance. However, at low lactose levels, the inclusion of inulin improved both piglet growth rate and intestinal health through a reduction in intestinal pH, increases in villous height, increased lactobacilli and increased short chained fatty acid concentrations.
Further research by Pierce et al., (2005c) found that the beneficial effects of inulin on the gastrointestinal health of piglets were evident when combined with lactic acid. The inclusion of both inulin and lactic acid in the diet increased the concentrations of lactobacilli in the colon. As lactic acid and inulin act on different regions of the gastrointestinal tract, supplementation of starter diets with both inulin and lactic acid should result in improved intestinal health along the entire length of the digestive tract.
Non-digested oligosaccharides such as mannan-oligosaccharides, fructo-oligosaccharides, galacto-oligosaccharides, and others can limit the population of pathogenic bacteria in the gut and consequently, improve health of the animal. Mannan-oligosaccharides bind to lectins on the cell walls of pathogenic bacteria like E. coli, thus preventing them from binding to and colonizing epithelial tissue (Pettigrew, 2000). The pathogenic bacteria-oligosaccharide complex then harmlessly passes through the digestive tract and out of the animal.
Cereal β-glucans
In pigs, cereal β-glucans resist hydrolysis by mammalian alimentary enzymes but can be hydrolyzed and fermented by gastrointestinal micro flora through the secretion of hydrolases in the front and hindgut. Recent research from our research group has shown the prebiotic effect of cereal ß-glucans as selective substrates for beneficial bacteria, particularly of the Lactobacillus and Bifidobacteria spp, as well as increasing the production of short chain fatty acids (O’Connell et al., 2005) .
Lynch et al., (2007) reported that there was a linear decrease in branch chained fatty acids, caecal Enterobacteria spp and increased colonic butyrate proportion as the level of barley increased in the diet. These authors suggested that 50 g/day of barley β-glucans is necessary in order to reduce odorous compounds in pig houses and to improve gut health.
Removing the beneficial effects of ß-glucans through enzymes
The addition of enzyme preparations can effectively degrade viscous NSP in the diet leading to a reduction in the viscosity of the diet and in enhanced nutrient utilization. However, the beneficial effects of fermentable fiber may also be removed. In two recent experiments (O’ Connell et al., 2005, Garry et al., 2007) our research group hypothesed that the beneficial properties of β-glucan in barley would be removed using an exogenous enzyme supplement resulting in a negative effect on gut micro flora and on odor and ammonia emissions.
In conclusion, a healthy diet needs to contain compounds that are not digestible by the host, but can be metabolised by at least some representatives of the gastro intestinal tract bacteria, preferably the LAB.
