For many years, the intestinal health of broilers was supported and preserved by the use of antimicrobial growth promoters (AGPs), antibiotic substances that, added to the feed in certain amounts, have subtherapeutic effects that favor animals’ productive performance. In 2006, the European Union banned the use of these substances as a precautionary measure to prevent AGPs from increasing antimicrobial resistance in bacteria and their subsequent transmission to humans. It is very likely that intestinal health problems in chickens were partially masked by the routine use of AGPs. Since their ban in Europe, alternative strategies have been sought to control the intestinal health of birds. These research efforts have revealed the complexity of the intestinal ecosystem.
Short chain fatty acids (SCFAs) have been widely used as feed additives in recent years. SCFAs are a group of molecules that contain from one to seven carbon atoms and that exist as straight or branched chain compounds. Among them, acetic, propionic and butyric acid are the predominant forms. Butyric acid also has multiple other health-promoting effects, such as mediating the immune response, inhibiting the growth of harmful bacteria, promoting the development of intestinal mucosal epithelial cells (by inducing proliferation and differentiation), and protecting epithelial cells. Butyrate is often available as a Na, K, Mg, or Ca salt. The advantage of this form over the free acid is that the salts are usually solid, nonvolatile, and odorless, and therefore easier to handle during the feed manufacturing process.
A correlation was found between the presence of butyrate and the control of pathogens such as Salmonella enteritidis and Clostridium perfringens. Butyrate significantly reduces Salmonella colonization and excretion in chickens and pathogen invasion into epithelial cell lines. In addition to the effect on Salmonella, butyrate can also influence the comparison of necrotic enteritis (NE) caused by C. perfringens. NE is a widespread and economically devastating bacterial disease in broiler farms that occurs in two forms. The subclinical form is characterized by poor performance (reduced growth, reduced food efficiency) without mortality, while the clinical form manifests itself with clinical signs and mortality. Butyrate does not have a significant antimicrobial effect against C. perfringens, but in an in vivo model it has been shown to reduce the number of animals developing necrotic lesions in the small intestine.
Finally, different studies evaluated the effect of unprotected or fat-coated butyrate and butyrate glycerides on broiler performance and showed their ability to beneficially influence feed conversion ratio (FCR) and body weight gain (BWG), intestinal villus structure and carcass quality, suggesting that butyrate and its derivatives may serve as a possible alternative to antimicrobial growth promoters.
The composition of the microbiota in the gut of chickens has a temporal and spatial connotation: the diversity of the microbiota increases with age and its composition differs between segments of the gastrointestinal tract. In general, a reduced number of bacteria is found in the proximal parts of the intestine, while the number increases towards the distal ileum and cecum. Furthermore, the diversity increases significantly towards the distal intestine. While only limited diversity can be seen in the small intestine, with lactobacilli often dominant, the ceca harbor many different bacterial groups. The microbiome of the cecum of healthy animals is dominated by bacteria belonging to the phyla Firmicutes (up to 75%) and Bacteroidetes (between 10% and 50%). Most of the sequences belonging to the Firmicutes phylum belong to the families Ruminococcaceae and Lachnospiraceas, respectively called groups IV and XIVa of Clostridium. Both families are producers of butyrate and, when present in a high enough concentration, the integrity of the epithelial barrier becomes stronger, the epithelial cells proliferate more and, therefore, the villi are longer. In addition, inflammatory reactions are reduced, while the stimulation of T lymphocytes generates a state of tolerance towards non-harmful bacteria.
The variety of metabolic functions of the gut microbiome encompasses the degradation of complex substrates, the fermentation of substrates to produce acidic compounds, immunomodulation, communication with other bacteria, and much more. To break down complex substrates, bacteria form a network, with different species and strains involved in the different steps of the substrate utilization process. The system by which bacteria convert substrates into products that are then converted by other bacteria is called cross-feeding. This important mechanism underlines the importance of having a bacterial variety within the microbiome that can promote the gradual degradation of substrates. In the intestine there are bacteria that promote butyrate synthesis through the production of intermediate metabolites, as well as bacteria that inhibit its synthesis by competing for the same substrate. For example, the mechanism by which lactic acid bacteria (LABs) may be beneficial can be explained by the effects of butyrate, considering that the lactic acid produced by these bacteria is then consumed by Clostridium group XIVa strains to produce butyrate. Lactic acid in high concentrations is toxic, and is only beneficial when it is converted. Sulfate-reducing bacteria compete for lactate with Clostridium group XIVa butyrate-producing bacteria, and the outcome of this competition is a crucial factor for intestinal health. Therefore, it can be deduced that there is a complex interaction between the different bacterial populations that make up the microbiome, and the result of these interactions can lead the different microorganisms to produce beneficial metabolites that favor intestinal health, or to produce toxic metabolites, harmful to the body.
Stimulation of endogenous butyrate production
Butyrate provides a link between diet, gut microbiota, and metabolic health. As described above, butyrate has several beneficial effects on gut health. Thanks to these beneficial effects, a strategy is sought to stimulate the production of butyrate in the intestine. This can be done using butyrogenic probiotics and prebiotics.
Probiotics. Most probiotic bacteria consist of LABs, mainly from the genera Lactobacillus, Bifidobacterium, Enterococcus, and Streptococcus. As discussed in the previous paragraph, lactic acid can be consumed by butyrate-producing bacteria. Different types of products are currently available in the market. In addition to single strains, multi-strain products are available, as well as competitive exclusion products, which contain a freeze-dried mixture of intestinal contents. The main purpose of competitive exclusion products is to replace the natural succession pathway of the microbiota, while probiotics enhance the functions of the existing microbiota.
There are no specific studies available in the literature showing the beneficial effects of Clostridium groups IV and XIVa butyrate-producing bacteria as probiotic candidates in poultry. However, Clostridium butyricum (group I), administered as a component of a three-strain probiotic, significantly improved body weight gain and reduced feed conversion ratio in broilers. Similar results have been obtained with C. butyricum supplementation in feed as a single strain probiotic. However, the main problem in the production of butyrate-producing bacteria as probiotics is the culture since these microorganisms are strict anaerobes. Another problem is that most poultry feeds are produced in pellet form and the probiotic strains are exposed to high temperatures during this process. Therefore, these probiotic strains are expected to be thermostable, for example, through spore formation. Unfortunately, very few strains of butyrate-producing bacteria appear to have this characteristic.