Synbiotics and gut health

Gut is the largest immune organ of the body (~69”) which contains around 70% of immune cells in the body. Feed cost remains ~70% of the total cost of production and feed is the most important entity that can expose the birds to a wide variety of factors through gut. During incubation, embryonic small intestine the weight increases faster than body weight. Till 17 day of incubation weight of small intestine is around 1%, which is 3.5% of body weight at hatching. Intestine of newly hatched chick is immature and undergoes morphological, biochemical, enzymatic & absorptive changes during first 14 days post hatch but first 24 hours are crucial for gut development. Access to feed and water within 6-10 hours of hatching will help in development of microvilli and has great impact on improving absorptive capacity of intestine.

Fig. 1. Effect of genetic selection on breast meat yield in broilers

Poultry GIT has a diverse community of bacteria, virus, fungi and protozoa, called as gut microbiota but the quorum is dominated by bacteria. The mature gut hosts around 800 species of bacteria whereas, a day old chick doesn’t hatch with established microflora. Gut microbiota consumes around ~20% dietary energy and includes Lactobacillus, Clostridium, Bifidobacterium, Bacteroides, Enterococcus, Streptococcus, Firmicutes, Proteobacteria and others. Major harmful gut bacteria are Salmonella, Clostridium and E. coli. Bacterial densities in the ileum & cecum 8 10 of the broiler chicks may be 10 & 10 cells per gram ingesta respectively at day 1 9 11 and reach up to 10 & 10 per gram of ingesta respectively within a week and remain stable till 30 days unless disturbed by any antimicrobials/pathogens.

Fig. 2. Effect of Bacterial Population in the GIT

Bursa and Thymus are primary immune organs where immune cells' production and differentiation occur. Functional immune cells leave the primary immune organs and populate secondary lymphoid organs like Spleen, Bone Marrow, Harderian gland and Bronchus Associated Lymphoid Tissue (BALT) & Gut Associated Lymphoid Tissue (GALT) (Ciriaco et al., 2003).

Major lymphoid organs in gut are GALT, Bursa of Fabricius, Cecal Tonsils, Peyer's patches and lymphoid aggregates in urodeum and proctodeum. GALT represents a component of Mucosa Associated Lymphoid Tissue (MALT) which includes bronchial, salivary, nasopharyngeal and genito-urinary lymphoid tissue and acts as first line of defence on mucosal surface (Suzan and Jeurissen, 1989).

Dysbacteriosis is an imbalance in the gut microbiota as a consequence of an intestinal disruption. This results in poor nutrient absorption in the gut leading to wet litter, excessive ammonia production and subsequently poorer FCR and reduced body weight. It is generally characterized by thinning of the gut wall along with gassy and watery gut contents. Dysbacteriosis can result from environmental stress, poor ventilation, inferior brooding conditions, viral or bacterial challenge, coccidiosis, poor biosecurity or in response to feed change.

Fig. 3. Dysbacteriosis in Broiler and Layer Farms

Probiotics and Prebiotics used in synergistic combination are termed Synbiotics.

Synbiotics are mixtures that improve the survival and implantation of live microbial dietary supplements in the tract, either by stimulating growth or by metabolically activating the health promoting bacteria.

Synbiotics have synergistic effects because they

Probiotics are defined as “live micro-organisms which when administered in adequate amounts confer a health benefit on the host” (WHO, 2001). Commonly used probiotics are spore forming & non-spore forming bacteria and yeast. Among noncolonizing species the most preferred one is Bacillus subtilis, as it has a tough, protective endospore, is a facultative anaerobe, is thermostable at pelleting temperatures and species like Lactobacillus, Bifidobacterium, Enterococcus, Streptococcus, etc. are good colonizers in vegetative form and available with or without microencapsulation.

Fig. 4. Bacillus subtilis

The yeast, Saccharomyces cerevisiae is another commonly used probiotic. Probiotics may affect quorum sensing in pathogenic bacteria, thus influencing their pathogenicity and eventually preventing GIT colonization (Medellin-Pena et al., 2007). B. subtilis spores vegetate in the intestinal lumen, consume oxygen, produce a more aerobic condition favourable for native Lactobacilli, which produce lactic acid, ultimately controlling pathogens.

The changes induced by probiotics in the intestinal epithelium are accentuated by the decrease in luminal pH, antimicrobial activity and secretion of antimicrobial peptides inhibiting bacterial invasion and blocking the adhesion to epithelial cells. In this sense, they improve the intestinal barrier by maintaining tight junction proteins, elevating the production of cytokines (TNF-α, IFN-γ, IL-10 and IL-12) (Arvola et al., 1999), which in turn, induce the secretion of IgA in the intestinal mucosa, causing the release of mucins (Gupta and Garg, 2009).

Fig. 5. Modes of action of probiotics

Mucins, the layer of glycoproteins that when in contact with water, form a film that lubricates and protects the intestinal epithelium against pathogens, forming a physical barrier between the epithelium and the content from the intestinal lumen (OliveiraSequeira et al., 2008), keeping the bacteria in a safe place in the intestinal lumen (Mattar et al., 2002). In the intestine, probiotics interact with enterocytes, goblet cells, M cells from Peyer´s patches forming GALT (Gut Associated Lymphoid Tissue) and these interactions result in an increase in the number of IgM and IgA contributing to the barrier against pathogenic micro-organisms (Szajewska et al., 2001). In addition, there is an antagonist effect through the secretion of substances that inhibit the growth and development of pathogenic bacteria such as bacteriocins, organic acids and hydrogen peroxide (Patterson and Burkholder, 2003; Oumer et al., 2001; Mazmanian et al., 2008).

Fig. 6. Maintenance of tight junction proteins (Mao et al., 1996)

Reduction of fat & triacylglycerol in liver, carcass and blood serum (Santoso et al.,1995), reduction in abdominal fat content (Delni et al., 2003) and significant increase in protein content in meat (Kalavathy et al., 2003). Significant decrease in bone/egg shell abnormalities (~10 %) by increasing digestibility, bioavailability and deposition of calcium, phosphorus and other minerals (Panda et al., 2006). Improve keeping quality of meat after slaughter and during freezing (Kabir, 2009).

Prebiotics are non-digestible food ingredients which beneficially affect the host selectively stimulating the growth and/or activity of one or limited number of bacteria in the digestive system and thus improve the host health (Gibson and Roberfroid, 1995). Fructo oligosaccharides (FOS), Galacto oligosaccharides (GOS), and Mannan oligosaccharides (MOS) are commonly employed prebiotics in poultry (Ferreira et al., 2011).

MOS is an outer cell wall protein, also called functional carbohydrate derived from the cell wall of Saccharomyces cerevisiae yeast (Kaplan and Hutkins, 2000). Due to beta linkages it can't be digested by endogenous enzymatic secretions and the only way is microbial digestion. From a food safety perspective, prebiotics are considered preventive agents and can be broadly categorized as either agents who reduce or eliminate already colonized food borne pathogens or prevent colonization of incoming pathogens. The intestinal mucosa has polysaccharide structure on their surface that is compatible to lectins.

Fig. 7. Ultrastructure of Saccharomyces cerevisiae yeast cell

E. coli, Salmonella contain lectins on their surface which aids them to get attached to the intestinal epithelial cells and this will be prevented by MOS (Markovik et al., 1997). Yeast mannans have also been shown to act as immune adjuvants and directly initiate immune responses by binding to macrophages and dendritic antigen presenting cells that contain the C-type lectins of the mannose receptor (Sheng, 2006).

Fig. 8. Benefits of Mannan Oligosaccharide (MOS)

Fig. 9. Role of β-glucan in gut immunity

After absorption, macrophages break down β-glucans into smaller particles and release them for number of days. Without β-glucans, macrophages will take longer time to digest pathogens. Activated macrophages will work more efficiently and kills intracellular and extracellular pathogens at a quicker rate. These activated heterophils will travel in blood stream and effectively control pathogens. There will be improved immune status of the birds and that will reflect positively in all the production parameters.

With the shift away from the routine use of antibiotics in poultry production, interest has grown in alternative feed supplements. Synbiotics in various forms offer a means to modify the GIT microbiota to benefit the bird in multiple ways. There is evidence that the GIT microbiota develop relatively early in the young chick's life cycle, perhaps even in ovo (Ilina et al., 2016). Addition of prebiotics in ovo has been explored and this may be a promising means to achieve greater consistency in prebiotic efficacy and immune function modulation (Madej et al., 2015). There are numerous resources still to be explored as alternatives for the development of poultry husbandry.