A balanced microbiota is vital for the intestine to have good function, in addition to gut integrity. The microbiota is formed by non-pathogenic microorganisms, which are of key importance for the metabolism and the absorption of nutrients and other compounds consumed by animals. A healthy microbiota has a high diversity of microbial genera in perfect balance, allowing an increase in the metabolic capacity of the intestine.
Beneficial microbiota assists in the digestion and absorption of nutrients, produces vitamins used by the host and decreases the proliferation of pathogens through exclusive competition and/or release of substances. This native microbiota produces short-chain fatty acids and lactic acid. These organic acids decrease the pH of the excreta, favoring the inhibition of pathogenic bacteria and stimulating the proliferation of enterocytes. As a result, cell structures and integrity can improve, which increases the absorption capacity of nutrients and consequently improves performance. Pathogenic bacteria, in turn, cause inflammation in the intestinal mucosa, generate toxic metabolites and promote the appearance of diseases, such as Salmonella, E. coli, Clostridium, Staphylococcus, Pseudomonas.
The population dynamics of the gut microbiota can be changed according to age, nutrition, stress, bacterial infectious diseases, hygiene and exposure to antimicrobials. When dysbiosis occurs, i.e., the imbalance of the microbiota with an increase in pathogenic bacteria, a reduction in the absorption of nutrients can occur, as well as an increase in the thickness of the mucosa and the speed of passage of the digesta. There is also an increase in the production of biogenic amines (cadaverine, histamine, putrescine etc.), ammonia and gases that are extremely harmful to the integrity of the mucosa and gut health.
The gut microbiota of animals also plays an important role in regulating the response of the immune system, since, in addition to modulating various physiological processes, nutrition, metabolism and exclusion of pathogens, it can change the pathophysiology of diseases, conferring resistance, or promote enteric parasitic infections. Natural bacteria in the intestine act as molecular adjuvants that provide indirect immunostimulation, helping the body to fight against infections.
Several additives are able to positively or negatively modulate intestinal function and consequently, influence animal health and performance. Some of them act as microbiota
modulators of the immune system and gut integrity, called nutraceuticals compounds; they include prebiotics, probiotics, organic acids, symbiotics, exogenous enzymes, polyunsaturated fatty acids and phytobiotics (Huyghebaert et al., 2011). Among them, the yeast stands out, which, through its compounds, changes the constitution of the microbiota by various mechanisms, modulates the immune response, the production of enzymes, vitamins and other metabolites that affect intestinal bacteria when used as prebiotics.
The Saccharomyces cerevisiae yeast cell wall (ImmunoWall®, ICC Brazil) – derived from the sugarcane fermentation process for ethanol production – contains around 35% of β- glucans (1,3 and 1,6), and 20% mannanoligosaccharides (MOS). β-glucans are recognized by phagocytic cells (Petravic-Tominac et al., 2010), stimulating them to produce cytokines that will initiate a chain reaction to induce immunomodulation and improve the responsiveness of the innate immune system. On the other hand, MOS is able of agglutinating pathogens which have fimbriae, such as different strains of Salmonella and Escherichia coli.
A recent study by Beirão et al. (2020, in press), in which broilers were fed diets supplemented with ImmunoWall® (0.5 kg/ton) and infected at two days of age with Salmonella enteritidis [SE] (orally, at a dosage of 10 8 CFU/broiler), showed changes in the microbiota and histopathology of the ileum and cecum. The treatments used in the study were: control (without additives and without challenge); challenge with Salmonella (10 8 CFU/broiler); ImmunoWall 0.5 kg/ton (without challenge); and Salmonella challenge supplemented with ImmunoWall 0.5 kg/ton.
Results showed a strong influence of ImmunoWall on the microbial population of broilers, such as the Subdoligranulum and Lactobacillus genera, and the Proteobacteria and Tenericutes phyla, as well as the challenge influenced the Ruminococcus genus.
Outstanding genera at 14 to 21 months old:
Ruminococcus torques: It has a mucolytic function and is of special interest, as it correlates with better productivity in broilers (Torok et al., 2011). Its presence was lower in the group challenged with Salmonella than in the control group. Although it has not been evaluated, it is interesting to observe that the challenge may have affected productivity through microbiota, and that the treatment prevented this change.
Subdoligranulum: The expression of butyrate kinase, an enzyme involved in the last step of one of the four potential ways of producing butyrate (Polansky et al., 2016), was detected in this genus. Butyrate is a short-chain fatty acid and plays an important role in intestinal physiology, serving as an energy source for cecum and ileum epithelial cells and controlling pathogenic bacteria by changing the internal pH of pathogens (Roto et al., 2015).
Based on these features, the Subdoligranulum bacteria takes on a positive role, but this agent has been associated with worse feed conversion in broilers (Singh et al., 2012). Therefore, the reduction of this genus may have been a positive effect of the treatment, as the results showed that both treatments with ImmunoWall reduced the amount of these bacteria.
Lactobacillus: It produces short-chain fatty acids and bacteriocins that prevent the growth of pathogens. Its beneficial effects on the host’s gut health have been well reported,
including better vaccine antibody responses, better defense against Salmonella and greater productivity (weight gain and feed efficiency) (Loman and Tappenden, 2016; Torok et al., 2011; Yan et al., 2017). It is known that MOS and nucleotides from yeasts induce the increase of bacteria known to be beneficial, such as Lactobacillus (Mesa et al., 2017). Confirming the literature, at 14 days both treatments with ImmunoWall increased the proliferation of Lactobacillus.
Phylum at 21 days of age:
Proteobacteria: The treatment with ImmunoWall reduced the amount of this phylum compared to the control group.
Tenericutes: Both treatments with ImmunoWall increased the proliferation of the Tenericutes phylum at 21 days. This unprecedented finding may be of great relevance since the Tenericutes genus appears in a greater proportion in the intestines of broiler with a better feed conversion rate (Singh et al., 2012).
The beneficial changes found in the microbiota, such as the increase in Lactobacillus and Tenericutes, may account for improvements found in other systems. These bacteria can enhance immune responses and improve gut integrity. In the same study by Beirão et al. (2020, in press) the average lamina propria area, inflammatory cells in the lamina propria and goblet cells of the ileum and cecum were evaluated, thus allowing the evaluation of the predominant cellular response to the pathogen (Tables 1 and 2).
Table 1. Histopathology of the ileum villi of broilers at 14 days of age.
ImmunoWall improved the condition of the intestine challenged with SE, as we can see from the lamina propria area where most of the immune cells are found. The number of inflammatory cells (ILP) was reduced with ImmunoWall supplementation (Challenge vs. Challenge + ImmunoWall; Control vs. ImmunoWall). Goblet cells are one of the first non-specific defense barriers of the intestine, and according to the results, we found that ImmunoWall provided greater production in challenged
Table 2. Histopathology of the cecum villi of broilers at 14 days of age.
The supplementation with ImmunoWall reduced the number of inflammatory cells (ILP) (Challenge vs. Challenge + ImmunoWall; Control vs. ImmunoWall). It was also observed that ImmunoWall provided greater production of goblet cells in challenged animals.
The effects of yeasts on the microbiota do not always induce the same changes described, indicating that there are important variations in environments and products and in the conduction of tests. Thus, this beneficial effect can be an important feature of ImmunoWall.
A correct measures program, including balanced nutrition, vaccination, reduction of stress factors, good management practices and animal welfare can significantly decrease the incidence of immunosuppression. The addition of dietary additives in the feed, which act by modulating the microbiota and gut integrity, improves the defense response in the face of challenges.