The gut is considered as the natural habitat for a large and dynamic community of microbes that regulate not only the intestinal but also the systemic functions of the host (Oakley et al., 2016). It also acts as the major barrier that separates the largest interface between the external and the internal environment while still allowing molecules to be absorbed or secreted (Mwangi et al., 2010). Thus, the gut and its integral components are functionally dependent on the proper physiological development of the host.
Over the past two decades, this topic has gained even more interest in poultry production due to increasing demands for economic efficiency, animal welfare, food safety, reduction in environmental impacts, and a ban on or avoidance of antibiotic growth promoters (AGPs) use (Morgan, 2017). The exogenous enzymes are capable of reducing the variability in feed ingredients and enhancing the feed digestibility availing more nutrients for absorption and thus reducing digesta viscosity. Recently the enzyme culture has gained immense value in the poultry industry and this has led to the development of more potential enzyme combinations to target specific substrates in feed and also complement endogenous enzymes. The role of exogenous enzymes emphasized on protease and phytase on gut health in poultry are mainly depicted in the present article.
Factors affecting gut health impairment
The common aspects affecting broiler gut health are stress, exogenous infection, diet and water, etc. Recently, with the advancement of exogenous enzyme study, more studies have been conducted on the impairment factors of the intestinal health of broilers focusing on phytic acid, and non-starch polysaccharides (NSPs).
Phytic acid is a natural antioxidant found in the form of salts and is present in cereals, vegetables, nuts, and natural oils (Silva and Bracarense, 2016). Phytic acid forms insoluble salts with minerals, including phosphorus, calcium, zinc, magnesium, and copper. Phytic acid increases mucin (MUC) excretion and endogenous nutrient losses, which are hazardous to intestinal health (Onyango et al., 2009). Phytic acids also form complexes with proteins and other nutrients (Yuet al., 2012). According to Selle et al., (2012), the possible mechanism of action is that, at an acidic pH, a binary protein-phytate complex is formed where phytate can bind to the α-NH2 groups and side chains of the basic amino acids arginine (Arg), histidine (His) and lysine (Lys).
Phytase as Gut health solutions
Phytic acid interferes with the normal digestive process resulting in significant quantities of starch and protein entering the large intestine, stimulating putrefactive bacteria and reducing gut health. Due to the current scenario of volatile feed prices more unconventional feed sources are introduced in poultry feed thus diets have become more complex and variable. Thus, the amount and availability of undigested substrates, have increased, which has a direct impact on microbiota composition and populations of non-beneficial bacteria in the intestine.
The use of exogenous microbial phytase to release phytate-P for absorption is almost ubiquitous in poultry feed, thus reducing the environmental impact of poultry production and the requirements for costly dietary inorganic phosphorus supplementation. However, there is an additional bonus of phytase application in that it potentially has a direct impact on microbiota and hence improves gut health (Morgan et al., 2017). Phytase increases protein digestibility and reduces endogenous losses, which limits protein supply to the hindgut. Dahiya et al. (2007) and Drew et al. (2004) observed that undigested protein substrates act as predisposing factors for dysbacteriosis, particularly necrotic enteritis (NE), suggesting that phytase could possibly alleviate the prevalence and severity of NE.
According to Lumpkins et al. (2009), phytase reduced intestinal mucin mRNA abundance in broiler chickens; as Clostridium thrives on mucin, reduced mucin concentrations could correspond with a reduction in C. perfringens. Smulikowska et al. (2010) stated that phytase increased caecal acetate, which is an indicator of microbial activity modulation, and Ptak et al. (2015) observed that phytase supplementation increased total ileal bacterial counts, Lactobacillus spp., Enterococcus spp. & total SCFA, DL-lactate and acetic acid concentrations.
Proteases as gut health solutions
Protease is an important factor in protein digestion as it hydrolyses the less digestible proteins in animal feeds and breaks them down into more usable peptides. Improving dietary protein digestibility with a specific protease inclusion can reduce feed cost by allowing the use of lower crude protein feedstuffs with lesser quality amino acids, effectively lowering protein and digestible amino acids levels required from the feedstuffs up to 10%. Recent research in poultry and swine has shown that supplementing diets with a protease enzyme support gut health and optimizes animal performance compared to non-protease-supplemented diets. Understanding the anti-nutritional effect of indigestible proteins in the hindgut of the animal helps to explain this effect.
Unconventional protein sources, such as Rapeseed meal (RSM), Cottonseed Meal (CSM) and Corn Distillers Dried Grains with Solubles (DDGS) due to nutrient variability can lower quality protein levels and reduce digestibility. Supplementing exogenous protease enables poultry that lack adequate levels of endogenous enzymes to digest proteins in the diet, which reduces the flow of undigested protein and other anti-nutritional entering the large intestine. Glycinin and β-conglycinin are two main anti-nutritional factors.
These allergens can cause intestinal allergies in piglets, causing irreparable damage. Protease, especially alkaline protease, has shown excellent effects on hydrolysing soybean allergens (Yin et al 2021). This indigestible protein serves as a fermentation substrate for undesirable bacterial strains, such as Escherichia coli, Clostridium perfringens(C. perfringens), Salmonella and Campylobacter in the gut. The proteins will be used by these harmful bacteria as nutrients. These pathogenic bacteria can then increase in population, shifting the balance of intestinal microflora against beneficial strains. Pathogenic bacteria can also produce toxic components such as bacterial toxins, but also fermentation metabolites like biogenic amines, ammonia and volatile sulphur compounds.
All of which can be detrimental to the performance by favouring oxidative stress, intestinal inflammation and lesions increasing both mortality and morbidity rates. In the case of a high protein diet, hindgut proteins are broken down into ammonia and amines, thus increasing the pH and favouring bacterial growth, including C. perfringens. Protein sources such as fishmeal, meat and bone meal, and soybean meal may increase the chances of necrotic enteritis in poultry. These protein sources contain some amounts of indigestible protein and amino acids that accumulate in the ceca and serve as a substrate for C. perfringens to flourish. Therefore, protease can be an effective solution when looking at necrotic enteritis prevention measures.
Anti-nutritional factors impair smooth digestion such that significant quantities of undigested starch and/or protein enter the large intestine, stimulating the activity of putrefactive bacteria and pre-disposing the animal to intestinal disorders. As already evident from the studies, the supplementation of exogenous enzymes will improve small intestine digestion and as a result limit substrate availability in the hindgut, therefore mitigating any potential GIT microbial dysfunction. There is still scope to explore the effect of supplementation of enzyme combinations targeting feed composition on gut health and ultimately impacting growth and efficiency.
References are available upon request