It is well known that bacterial fermentation occurring mostly in the large intestine of pigs, as well as its consequent effect on their intestinal "ecosystem", is mainly determined by the carbohydrates in the diet that have escaped from digestion and absorption in the small intestine. However, the effect of bacterial fermentation of the diet protein that also escapes digestion and reaches the large intestine undigested, on the intestinal health of pigs is less well known. This undigested protein also constitutes an important substrate for the fermentative processes that take place in the large intestine of pigs. The amount of fermentable protein depends on factors such as the level of protein in the diet, its digestibility, interaction with other components of the diet or with antinutritional factors, and the secretion of endogenous proteins in the intestinal lumen. There is a wide variety of bacterial species with a great capacity to ferment undigested protein reaching the back of the gastrointestinal tract, such as Escherichia coli, Klebsiella spp., Campylobacter spp., Streptococcus spp., Clostridium perfringens, Clostridium difficile and Bacteroides fragilis. This variety of bacteria would explain why diets with high levels of protein are usually associated with intestinal dysbiosis and enteric processes involving diarrhoea.
In addition to short-chain fatty acids —also derived from the fermentation of carbohydrates— protein fermentation gives leads to the formation of branched-chain fatty acids from branched amino acids. However, proteolytic fermentation in the large intestine also generates potentially toxic metabolites such as ammonia and certain amines (histamine, tyramine, diamines) and polyamines (putrescine, spermine and spermidine), as well as phenolic compounds such as cresol, indole and skatole from aromatic amino acids. Finally, the sulfur amino acids and sulfomucins of the intestinal epithelium are transformed into sulfur-containing metabolites such as hydrogen sulfide. The negative effects of these metabolites produced during proteolytic fermentation are varied. Thus, for example, ammonia can interfere with the oxidative metabolism of short-chain fatty acids inside the epithelial cells, which could lead to energy deficiencies in these cells. Amines, such as histamine, lead to inflammation of the intestine and can cause the secretion of chlorine in the pigs' colon and, consequently, diarrhoea. On the other hand, phenolic compounds increase epithelial permeability and they appear to be involved in the production of toxic metabolites, such as nitrophenol and diazoquinone. As per hydrogen sulfide, its effects depend on its concentration in the intestine. High concentrations of this compound can negatively affect cellular respiration and damage the DNA at epithelial level, in addition to stimulating the secretion of chlorine. In short, a poor use of protein in the diet not only reduces the availability of amino acids for the pigs' growth and metabolism, but it also leads to intestinal dysbiosis, with proliferation of pathogenic bacteria and consequent epithelial damage, that will contribute to the development of enteric disorders.
Various strategies are followed, both at nutritional level (eg, reduction of crude protein levels in the diet and supplementation with amino acids) and that of antibiotic use, to prevent or mitigate the problems derived from proteolytic fermentation. Although the positive effect of the reduction of protein in the diet is well documented, recent scientific studies have shown the possible negative effect of early use of antibiotics in the diet on proteolytic fermentation in pigs. A work recently published in the scientific journal Anaerobe, conducted by Zhang et al. (2016), investigated the effects of early administration of antibiotics (oxytetracycline, oliquindox and kitasamycin) in feed on bacterial communities and proteolytic fermentation in the intestine of pigs fed two different levels of protein: a "normal" level (20% from 42 to 77 days and 18% from 77 to 120 days) and a low level (16% from 42 to 77 days and 14% from 77 to 120 days). The study revealed that early administration of antibiotics in the feed significantly reduced some beneficial bacterial populations, such as lactobacilli, in the medium term (77 days of life) and butyrate- producing Clostridium species in the long term (120 days of life) in pigs fed normal levels of protein. In addition, as indicated in the following table, the administration of antibiotics in the feed at an early age increased the production of certain metabolites of proteolytic fermentation at later ages, which could increase the risk of enteric diseases or, at the very least, compromise the pigs' optimal growth.
Table 1. Effect of early administration of antibiotics in the diet on proteolytic fermentation later in life (Zhang et al., 2016)
|Low protein level||Normal protein level|
|Age 70 days|
|Age 120 days|
Given the growing restrictions on the use of antibiotics in feed worldwide, surely strategies against bacterial dysbiosis of nutritional origin imply combining the use of low levels of protein in the diet with highly digestible protein sources and with certain probiotics (e.g., Bacillus licheniformis and Bacillus subtilis) that, in addition to regulating intestinal microbiota, synthesize and excrete digestive enzymes that increase digestibility of proteins in the diet.