The microbiome of the gastrointestinal (GI) tract of poultry is very diverse yet relatively stable in a dynamic state. The poultry (e.g. duck, chicken and turkey) GI tract consists of cloaca, colon, cecum, small intestines (duodenum, jejunum and ileum), gizzard, proventriculus, crop and esophagus. The GI tract of the poultry is much shorter than that of mammalian animals. But it contains highest bacterial abundance and diversity. The bacteria found in the intestine mostly include Escherichia coli, Lactobacillus, Bacteroids, Eubacterium, Peptostreptococcus, Propionibacterium as predominant organisms. Other group of micro organisms such as anaerobic, gram-negative cocci, facultative anaerobic cocci and streptococci are also found in the GI tract. In this article we briefly discuss the factors affecting the poultry gut micro biome and its importance for poultry nutrition.
Microbiome and Host
Many intestinal bacteria hydrolyze carbohydrates to simple sugars which are further fermented to short chain fatty acids (SCFA) (viz., butyrate, propionate and acetate) by other intestinal bacteria. The SCFA are utilized as a source of energy and carbon. Gut bacteria also contribute to host nitrogen metabolism. These bacteria metabolize uric acid to NH3, which is utilized by the host to synthesize a few amino acids such as glutamine. Gut micro biome of poultry may also serve as a source of vitamin to its host. Mucins secreted by goblet cells of the gut are important source of carbon, nitrogen and energy for some commensal and pathogenic bacteria. Gut micro biome also has impact on intestinal morphology of poultry. One such effect is evident when birds raised on a conventional diet show shorter intestinal villi and shallow crypts with low load of bacteria. However, dietary supplementation of probiotic organisms increases villus height: crypt depth ratio in ileum of broilers.
Microbiome and Immunity
The first line of defense mechanism in the inner surface of avian gut is the gel-like mucus layer formed from mucin glycoprotein produced by the goblet cells. The mucus layer prevents the intestinal pathogens from penetrating into intestinal epithelium. The disruption of the mucus layer is probably due to the severe necrosis of the intestinal mucosa which results in vast shedding of goblet cells. Production of beta-defensin is another important strategy present on the intestinal epithelial surface. Βeta-defensin are produced by avian macrophage, heterophils and epithelial cells that kills various intestinal pathogens by disrupting cell membrane permeability. In birds, the cell mediated immunity (T and B cells) can be found in dispersed areas (lamina propria and epithelium) and in more organized lymphoid tissues (Payer’s patches and bursa of fabricius).
Microbiome and Diet
Diet has great potential to modulate the host digestion and nutrient absorption. Wheat, barley or rye-based diets have more impact on the gut micro biome. These diets contain high levels of water-soluble, indigestible, non-starch polysaccharide that favor necrotic enteritis. Excessive non-starch polysaccharide leads to rise in digesta viscosity, decreased digesta passage rate and a decline in nutrient digestibility. Another potential diet ingredient, soyabean is used as a source of protein to promote the growth lactobacilli population and reduce the number of coliforms in cecum of poultry. Some of the gut micro organisms are also influenced by dietary fat source. Dietary enzymes such as xylanase and beta-glucanase, increase intestinal lactic acid bacteria(LAB) and decrease the population of adverse and pathogenic bacteria such as E. coli. Dietary supplementation with xylanase and beta-glucanase protects against necrotic enteritis as the enzyme breakdown the non starch polysaccharide in the diet and reduce the digesta viscosity. Plant derived trans-cinnamaldehyde and eugenol are effective in reducing S. enteritis colonization in 20-d old broiler chickens. Others such as blend of essential oils, containing thymol, carvacrol, eugenol, curcumin and piperin reduce the colonization and proliferation of such pathogens. Antibiotic growth promoter (AGP) is another feed additive used to improve feed efficiency, increase animal growth and maintain animal health. The inclusion of AGP in poultry diet reduces the incidence of disease and promotes better performance of the birds by inhibiting the growth of enteric pathogens. However, due to rising antibiotic resistance among the pathogens, the use of AGP has been prohibited. The proliferation of the bacteria present in the gut can be increased by the ingestion of prebiotics.
Prebiotics are polysaccharides such as galatosaccharide (GOS) and fructosaccharide (FOS).
GOS favors the growth of Bifidobacteria in the GI tract of broiler chicken.
Competition for nutrient and attachment site
The GI tract of newly hatched chick is sterile, but is immediately colonized by surrounding organisms. Over the period of time, normal colonization and succession of gut micro biome takes place in healthy adult poultry’s intestine. The GI tract serves as an ideal habitat for micro organisms however, due to limited space and resources; there is competition among organisms for nutrient resources. Some bacteria produce bacteriostatic or bactericidal substances to kill its competitors. The LAB ferment carbohydrates to organic acids and inhibits the growth of certain pathogens such as E. coli and Salmonella by reducing the pH of the gut. Certain bacteria such as Enterococcus sp., Pediococcus sps., Bacillus subtilis also produce antimicrobial agent called bacteriocins to selectively inhibit the growth of other bacteria. However, pathogens are adapted to new environment very fast mediated by a process such as conjugation, transformation and transduction. Providing probiotics (live microbial feed supplement) benefits the host through the following mechanisms:
(1) Inhibiting the growth of pathogenic bacteria from colonizing and proliferating in the gut through competition for nutrient and attachment site
(2) Production of bacteriostatic and bactericidal substances against pathogens
(3) Enhancing gut barrier function and
(4) Enhancing host immunity.
Poultry litter microorganisms influence gut microbiome
Chickens are in constant contact with the micro organisms from the surrounding environment. The poultry litter usually harbors a complex microbial community. Reuse of poultry litter commonly practiced by poultry farmers to reduce produce cost, influences chicken guts micro biome. The reused litter may also harbor disease-causing micro organisms from the previous flock and thus serves as a source of pathogens to the subsequent flock.
The gut represents an essential microbial ecosystem that lives in symbiosis with the host. The development of GI micro biome plays a crucial role in the nutrition, health and growth of the chicken. Thus further research on the intestinal micro biome of the poultry can potentially provide us more knowledge to improve management of poultry diseases, antibiotic resistance and better control of colonization and spread of human pathogens.