In general terms, a post-biotic agent is defined as a substance released or produced by the metabolism of a microorganism that generates a beneficial effect on the host, either directly or indirectly (Zólkiewicz et.al, 2020). In the last decade, postbiotics have gained recognition for their numerous advantages on the intestinal flora; and, since they are substances that do not contain live microorganisms, the risks associated with their ingestion are lower. Post-biotics have little or no interaction with the feed content, thereby they maintain sensory and physicochemical properties of the nutrients. It is a feed additive stable in a wide range of pH and temperature, enabling feed ingredients to be exposed to higher acidity and reducing the possibility of microbial contamination during feed storage (Pérez et.al, 2022).
Below is a brief description of the classes of postbiotics.
Cell-free supernatants
They are substances that contain biologically active metabolites secreted by bacteria and yeasts into a surrounding fluid. These substrates can be extracted directly from cell cultures and each culture of microorganisms will show different activities or actions. In relation to yeasts, the supernatants of Saccharomyces cerevisiae and Saccharomyces boulardii have a positive effect in reversing the alteration of intestinal peristalsis caused by stress stimuli. In turn, S. boulardi has anti-inflammatory and antioxidant activity (De Marco et.al, 2018). It is worth mentioning that yeast supernatants are similar to bacterial ones, in terms of accelerating the wound-healing process and the regeneration of the intestinal barrier (Zólkiewicz et.al, 2020).
Exopolysaccharides
As it is well known, microorganisms have the ability to form biopolymers. These elements are released outside the bacterial cell wall and make up a heterogeneous group of substances called exopolysaccharides. The obtained group works as stabilizing, emulsifying and water-binding agents (Singh & Saini, 2017). In the field of exopolysaccharides, B-glucans are included, which improve the immune response against bacteria, viruses, parasites and neoplastic cells. The benefits of the use of exopolysaccharides not only cover the issue of immunity, but also influence the efficacy of probiotics, contributing to the adhesion of the microbiota in the intestinal epithelium. With its application, the bioavailability and absorption of carotenoids (substances that stand out for being antioxidants and anti-inflammatory) increases (Zólkiewicz et.al, 2020).
Enzymes and fragments of the cell wall
Several microorganisms are known for their ability to form defenses against reactive oxygen species (harmful to lipids, proteins, carbohydrates and nucleic acids). Usually, as a protective mechanism, enzymes are typically produced to mitigate the effects of ROS. The main enzymes are: glutathione peroxidase, peroxide dismutase, catalase, and NADH-oxidase. As for the components of the bacterial cell wall, they are mainly immunogenic (specific immune response), including bacterial lipoteichoic acid. It lies on gram-positive cell walls and can be released spontaneously into the environment (Zólkiewicz et.al, 2020).
Short-chain fatty acids
Short-chain fatty acids are a product obtained from the fermentation of vegetal polysaccharides from the intestinal microbiota. The main ones are acetic, propionic and butyric. Acetate comprises a role in the direct regulation of appetite via the central nervous system. Propionate is used as a substrate of gluconeogenesis in the liver to obtain energy. In addition, it inhibits the synthesis of cholesterol and has anti-inflammatory activity. Finally, butyrate, is the most important energy source of enterocytes, promoting the renewal of the intestinal epithelium and inducing tolerance to food (it increases the expression of immunosuppressive cytokines and regulates proinflammatory receptors).
Bacterial lysates and metabolites produced by the intestinal microbiota
The bacterial lysates are obtained by chemical or mechanical degradation of gram-positive and negative bacteria found in the environment. The bacterial components are used clinically based on the concept of the gut-lung axis, which refers to the functional connection between the gut immune system and the respiratory system. The mechanism of action of the bacterial lysates is to generate an immune response in Peyer’s patches of the small intestine, promoting the formation of mature lymphocytes that then migrate to the respiratory tract. In this way, the proliferating lymphocytes become involved in subsequent innate immune responses and secrete an extensive amount of IgA (Kearney, Dziekiewicz, & Feleszko, 2015). On the other hand, the intestinal microbiota is responsible for producing a variety of molecules such as vitamins, metabolites derived from phenolic compounds and aromatic amino acids; that are extracted and integrated as a postbiotic agent (Zólkiewicz et.al, 2020).
There are not enough data available to understand the complexity of functions around postbiotics, so their effect is pleiotropic. Postbiotics have immunomodulatory, antitumor, antiatherosclerotic, healing, and infection prevention effects (Zólkiewicz et.al, 2020).
Hydrolyzed Yeast is a new product derived from a eukaryotic microorganism that is used as food material and is obtained by different strategies such as autolysis or enzymatic hydrolysis. Autolysis in particular is a degradative process, where intracellular enzymes are used to solubilize cellular elements inside the cell. For its part, enzymatic hydrolysis uses digestive enzymes that break the cell wall. With any of these methods, a set of residual elements of the cell lysis process is obtained, among which are: B vitamins, nucleotides, amino acids, mannan oligosaccharides and B-glucans. The choice of the method by which the hydrolyzed yeast will be obtained depends closely on the recovery time and yield. The enzymatic hydrolysis process is significantly faster and with higher yield than other processes (Perricone et.al, 2022).
Few reports mention the use of hydrolyzed yeast in animal production. The vast majority of literature is described in poultry farming.
In poultry, the effectiveness of the hydrolyzed yeast varies according to the rearing stages, having a better performance at the beginning of the cycle. It generally contributes to growth, increasing body weight gain and improving the feed conversion rate. Breeding and laying hens present a higher productive performance, in addition to transferring their positive effects to the progeny (Araujo et.al, 2018). Hydrolyzed yeast is also associated with prebiotic and probiotic activities on intestinal microorganisms, resulting in a healthy intestine that efficiently uses food (Perricone et.al, 2022). In the intestine of birds, it has been reported that components of the yeast wall, such as mannan, carry out a selection of harmful bacteria for the organism such as Escherichia coli or Salmonella, and prevent them from attacking and destroying the cells of the intestinal epithelium ( Peralta, Miazzo, & Nilson, 2008). In pigs, some studies corroborate the positive effects of yeast derivatives, mitigating post-weaning diarrhea caused by E. coli (White, Newman, Cromwell, & Lindemann, 2002; Kogan & Kocher, 2007). Finally, an investigation conducted on weaned calves found that the components of the yeast wall can increase their growth and at the same time enrich their gastrointestinal physiology (Ma et.al, 2020).
In conclusion, postbiotic agents are metabolites originated from microorganisms that play a beneficial role, both intestinal and systemic. As they are not made up of live microbes, the risk of their application is minimal. In animal health, hydrolyzed yeast is one of the most widely used postbiotics and one with the greatest projection into the future. The successful result of the hydrolysis of the yeast wall determines the obtaining of structural components that function adequately in the prevention and control of pathogens that enter via the gastrointestinal tract, in addition to promoting immunity. Its application, as well as the number of research reports, remains still limited in the veterinary field.
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