Mannan oligosaccharide supplementation may improve broiler performance through modulation of the intestinal microbial populations and exclusion of pathogens. It has been demonstrated that pathogens possessing type 1 fimbriae are much more virulent than non- fimbriaed bacteria (Duguid et al., 1976; Connell et al., 1996; Gunther et al., 2002). Type 1 fimbriae are expressed in Escherichia coli, Salmonella as well as most other species of bacteria in the Enterobacteriaceae family (Duguid and Old, 1980; Jones et al., 1995; Duncan et al., 2005). Mannose-specific type-1 fimbriae present in bacteria allow them to attach to the receptors
containing D-mannose (Eshdat et al., 1978) in the intestinal epithelial lining. Attachment of pathogenic microbes to the enterocyte cell wall is a prerequisite for bacterial colonization and the onset of infections (Gibbons and Vanhoute, 1975). However, the attachment of Gram negative
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bacteria can be inhibited by blocking their type-1 fimbriae with mannose or similar sugars. Thus, dietary inclusion of MOS may allow enteric pathogens to attach to the mannan compounds in the gut lumen instead of the epithelia, thereby reducing pathogen colonization and subsequent infections.
Reduction of Salmonella infection by MOS has been demonstrated in a number of studies (Fernandez et al., 2000; Spring et al., 2000; Fernandez et al., 2002). Spring et al. (2000) indicated that MOS decreased the number of salmonella positive chickens from 89.8% to 55.7% upon challenge with Salmonella Typhimurium and Salmonella Dublin. Similarly, some other in vivo and in vitro studies reported that mannose decreased the adherence and colonization of
Salmonella Typhimurium in the intestine of young chicks (Oyofo et al., 1989a; Oyofo et al., 1989b; Oyofo et al., 1989c). A reduction of mucosa-associated coliforms was also observed in the birds supplemented with MOS as early as 7 days of age (Yang et al., 2008a). Similar findings were observed by Iji et al. (2001c) and Santin et al. (2001) and the authors suggested that the reduced coliforms may contribute to the observed improvement in gut morphology and function. Ileal luminal coliform counts have also been shown to be reduced by MOS (Jamroz et al., 2003; Yang et al., 2008b).
Pathogenic bacteria may cause direct damage to the intestinal epithelium as well as elicit immune responses in birds. Immune response can manifest as fever and reduced appetite
mediated by acute phase cytokines (Johnson, 1997; Spurlock, 1997). Further, cytokines modulate nutrient metabolism including glucose, lipids, amino acids and minerals by coordinating muscle, adipose, bone and hepatic metabolism in favour of providing these nutrients in support of the immune response. Therefore, infection burdens are associated with growth retardation due to
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decreased food intake, impaired nutrient absorption from intestinal damage and increased metabolic requirements to support repair and immune response.
Prebiotics are defined as non-digestible feed ingredients which elicit a beneficial effect in gut health by selectively enhancing the metabolic activity or growth of beneficial intestinal microorganisms (e.g. bifidobacteria and lactobacilli) (Gibson and Roberfroid, 1995; Cummings and Macfarlane, 2002; Ohimain and Ofongo, 2012). Mannan oligosaccharides have been characterized as prebiotics for poultry based on the reports (Denev et al., 2005; Baurhoo et al., 2007a; Baurhoo et al., 2007b; Baurhoo et al., 2009) that dietary inclusion of MOS caused a significantly increased intestinal lactobacilli and bifidobacteria population than the broilers fed control diet . Although the exact mechanism is unclear, it was hypothesized that the decreasing number of viable Gram-negative bacteria (type 1 fimbriae-bearing) due to dietary MOS may increase the Gram-positive organisms in the gut. Lactobacilli and bifidobacteria are Gram- positive bacteria that include no pathogenic species and are suggested to maintain the integrity and health of the chicken gut by several modes of action including competing against potential pathogens for epithelial binding sites and nutrients, secreting bacteriocins (Gibson and Wang, 1994; Kawai et al., 2004) and modifying the synthesis and secretion of mucin in the gut (Smirnov et al., 2005). Moreover, lactate produced by lactobacilli and bifidobacteria during carbohydrate fermentation, may decrease the growth of the pathogenic bacteria such as Salmonella and E. coli, (Barrow, 1992). However, in broilers, the effect of MOS on intestinal lactobacilli and bifidobacteria is not consistent as reported throughout the literature (Spring et al., 2000; Fernandez et al., 2002; Biggs et al., 2007).
Increased villus height has been observed in several studies in which broiler chickens were fed MOS-supplemented diets (Loddi et al., 2002; Baurhoo et al., 2007b; Yang et al., 2007a;
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Baurhoo et al., 2009). An association between increased intestinal villus height and increased colonization of lactobacilli and bifidobacteria in MOS-fed broilers was identified in studies conducted by Baurhoo et al. (2007b) and Baurhoo et al. (2009). The effect of beneficial bacteria such as Lactobacillus sp. on increased villus height to crypt depth ratio in the duodenum and ileum has been reported in chicks (Awad et al., 2010). Additionally, it was found that the birds given MOS had shallow crypts compared to controls (Savage et al., 1997; Yang et al., 2008a) consistent with less turnover rate of the intestinal epithelium. An increased villus height: crypt depth ratio observed in birds supplemented with MOS (Ferket et al., 2002) may indicate reduced damage of enterocytes and therefore the need for cell renewal in the gut. As a result, there is less nutrient demand for maintenance of intestinal mucosa, which in turn favors faster growth rate of the whole animal.
The specific activities of digestive enzymes and gut morphology are indications of the rate of epithelial cell turnover and thus the maturity and functional capacity of enterocytes. It has been reported that specific activities of brush border enzymes are enhanced by MOS (Iji et al., 2001c; Sklan, 2001; Yang et al., 2008a). Significant increases in the specific activities of sucrase, maltase, and leucine aminopeptidase were detected in in the jejunum but not ileum of broiler chickens up to an inclusion level of 3g kg-1 MOS (Iji et al., 2001c).
MOS supplementation has also been reported to reduce some pathogens which do not possess type 1 fimbriae (i.e. coccidia, Clostridium perfringens and Campylobacter), but the mechanism remains unknown (Novak and Troche, 2007). Feed supplemented with MOS has been found to decrease Campylobacter colonization in broiler chickens (Shane, 2001; Anderson et al., 2005). Also, it has been documented that feeding birds with MOS may prevent
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MOS does not bind to these bacterial species, Ferket, (2004) suggested that those bacterial counts were reduced by MOS, possibly by enhancing the mucin barrier or stimulating gut associated immunity.