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The improvement of feed consumption and genetic selection have been the primary areas of poultry research. The control of a variety of microbial infectious diseases caused by Streptococcus, Staphylococcus, Escherichia coli, Pseudomonas, Campylobacter, Salmonella, Yersinia, Bacillus, Clostridium, Mycobacterium, Enterococcus, Klebsiella and Proteus species has been less thoroughly investigated. The immune system of broilers is not fully developed during the first few weeks and therefore it is more susceptible to bacterial infection. Furthermore, it can take up to eight weeks for the gut microbiota to develop and stabilize. The longer the time necessary to reach bacterial homeostasis, the greater the risk of bacterial infection. Poultry are kept in closed facilities to minimize the risk of bacterial infection.
The pathogen most prevalent in the intestines of chickens, particularly broilers, is Salmonella. Therefore, probiotics have been potential candidates for growth promoters in the majority of commercial poultry diets since the withdrawal of antibiotic growth enhancers in poultry nutrition. Antibiotic growth promoters act by blocking the production and secretion of pro-inflammatory cytokine-degrading intermediates in the gastrointestinal tract, resulting in the disturbed gut microbiota [1]. Probiotics, on the other hand, alter the gut environment and strengthen its barrier function through the immune system stimulation, as presented in Table 1 [2]. A total of 280 females of Japanese quails were fed with a mixture of without rapeseed meal, non-fermented post-extraction rapeseed meal (5%, 10%, 15%), and a fermented one (5%, 10%, 15%). The data analysis revealed that the addition of 10% fermented rapeseed meal had the most beneficial effects as such egg quality traits as egg weight, specific gravity, yolk index and color, and albumen pH [3]. In broilers, probiotic non-pathogenic bacteria compete with the pathogenic ones for nutrients in the gut. They also colonize the intestines, preventing pathogenic bacteria from inhabitation and stimulating the secretion of digestive enzymes (e.g., β-galactosidase, α amylase), thus facilitating the absorption of nutrients, and enhancing broiler growth performance [4]. An increased average dietary feed consumption and conversion result in an improved body weight gain, which in turn affects production performance, even though probiotics do not always significantly influence feed consumption and feed conversion. All of the above-mentioned processes depend on several factors, including strain selection and application, time concentration, as well as the absorption of dietary probiotics [5]. An increased average dietary feed consumption and an enhanced feed conversion efficiency are closely attributed to an improvement in body weight gain, which in turn complements production performance [6]. The application of dietary supplementation of probiotics improves body weight gain and feed conversion, even though probiotics do not always significantly enhance feed consumption [7]. The body weight gain, average daily diet consumption, feed conversion efficiency, and production performance of the poultry birds are influenced by several potential factors including strains selection, application, time concentration, and absorption of dietary supplementation of probiotics [8].
Table 1. Summary of the beneficial probiotics on poultry performance.
Probiotic Strains | Biological Performance | Reference |
---|---|---|
Bacillus amyloliquefaciencs | Improve intestinal health and growth performance | [9][10] |
Bacillus coagulans | Enhances growth performance and gut health | [11] |
Lactobacillus acidophillus | Improve production performance and helps the immune system and gut histomorphology | [12][13] |
Lactobacillus bulgaricus | Enhances growth performances and improves immune functions | [14] |
Pediococcus acidilactici | Improves laying performances and modulates intestinal microflora composition | [15][16] |
Propionibacterium acidipropionic | Contributes to the better development of intestinal mucosa and microbiota composition | [17] |
Saccharomyces cerevisiae | Improves growth performance and enhances laying performance | [18] |
Streptococcus faecium | Avoided the impairment and regulated the stability of the epithelial intestine, and improves the immune functions | [19] |
Wang et al. [20] immunized hatched chicks with a strain of Lactiplantibacillus plantarum LT-113, and found that it protected against Salmonella typhimurium by limiting gastrointestinal invasion and inhibiting tight junction gene expression in intestinal cells. In the control group, Salmonella infection compromised the intestinal mucosal barrier. On the other hand, Olnood et al. [21] revealed that the oral administration of Lactobacillus johnsonii decreased Salmonella and Clostridium perfringens invasion in the gastrointestinal system. Additionally, it has been demonstrated that the combination of xylanase and a multistrain probiotic enhanced dietary energy absorption in the intestine and its preservation in the liver [22]. Energy changes may result from improved nutrient digestibility and feed conversion rate. Probiotics increased the synthesis of short-chain fatty acids, stimulated the immune system and metabolism [23][24].
Short-chain fatty acid (SCFA) metabolites produced during microbial carbohydrate fermentation in the gastrointestinal tract impact leukocytes and endothelial cells by stimulating G-protein-coupled receptors and inhibiting histone deacetylase. SCFAs increase the level of IgA produced by B immune cells, impede the NF-κB transcription factors and reduce the production of proinflammatory cytokines [25][26]. Dietary supplementation of poultry with Bacillus licheniformis, a facultative anaerobic bacterium, enhances the gastrointestinal tract absorption rate and surface area. It also stimulates the growth and multiplication of probiotic bacteria such as Lactobacillus, Bifidobacterium, and Aspergillus awamori, as shown in Figure 1 [27].
Zheng et al. [11] exposed broilers to Salmonella enteritidis (SE) and found a significant decline in goblet cell membranes at 7 days post-infection (DPI), as well as a reduction in villus height and villus-crypt ratio in the small intestine. In contrast, birds fed dietary supplementation of Bacillus coagulans had a relatively low crypt depth, a greater villus-crypt ratio, and a larger number of goblet cells in the jejunum at 7 and 17 days post-infection. Gastrointestinal mucous cells synthesize mucin-2, a constituent of mucus, which facilitates the enhancement of barrier activity in Salmonella enteritidis-infected birds. Supplementation with a Bacillus licheniformis-fermented product at 1.25 and 5 g/kg improved cecal morphology and increased the survival rate of broilers and conserve a stable number of goblet cells in the ileum as well as in the caecum under Eimeria tenella challenge. A 1.25 g/kg dose reduced lesions scores in the cecum, while that of 5 g/kg decreased the oocyst-count index. Furthermore, surfactin C isolated from Bacillus licheniformis-fermented products inhibited Eimeria oocyst sporulation and disrupted sporozoite morphology [37].