Probiotics as a Friendly Antibiotic Alternative: History
Please note this is an old version of this entry, which may differ significantly from the current revision.

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.

  • intestinal morphology
  • reproduction
  • antibiotic resistance
  • poultry

1. Probiotics and Growth Performance

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].

Figure 1. Effect of probiotics on growth performance, gut macroflora and feed efficiency.

2. Probiotics and Intestinal Morphology

Poultry health status and improved growth efficiency are strongly correlated with the gut condition and intestinal microflora. The intestines play a very important role in the digestive tract of birds as they harbor a diverse community of beneficial microbes, which degrade complex nutrient compounds into simpler molecules that are more easily assimilated and metabolized [28][29]. The structural organization and adherence properties of the gastrointestinal epithelial cells are crucial for nutrient absorption and the protection of the bird’s body from pathogenic microbes that could infiltrate the bloodstream [30]. The most important parameters associated with the higher nutritional absorption resulting from larger surface area available for nutrient assimilation are those related to intestinal morphology, i.e., increased villus height, a lower crypt depth, and the higher villus height to crypt depth ratio [31]. Sound gastrointestinal microbiota is the prime requirement for avoiding microorganism infections in the gastrointestinal tract of birds. This is achieved by preventing microorganism colonization through pathogenic bacteria antagonism, inhibition of adhesion sites in the gut and impediment to bacterial exercises [32]. Similarly, another marker of gastrointestinal health status is the amount of gastric mucosa, which produces mucin and prevents pathogenic organisms from adhering to mucosal surfaces [33]. Even though the intestinal microflora is relatively stable, it is still affected by numerous environmental factors (feed composition, hygienic standards, physical stress, etc.) and overall health condition of the animal. However, the key element with the greatest impact on intestinal microflora is diet. Probiotics are commonly used for intestinal flora regulation [34] and improvement of gastrointestinal histomorphology; however, the potential effects may slightly differ from one strain to another [35]. Dietary supplementation of broilers with Lactobacillus plantarum and Lactobacillus reuteri significantly affected barrier activity and reduced colonization by certain opportunistic or pathogenic microorganisms [36].

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].

3. Probiotics and Immune Response

The chicken requires a strong immune system for optimal performance. The immune system comprises lymphoid organs located in the different parts of the body. In addition to highly specialized lymphoid structures like Meckel’s diverticulum, the bursa of Fabricius, cecal tonsils, and Peyer’s patches, which are connected to the gastrointestinal lumen, numerous lymphoid cells can be found in the epithelial mucous membrane (intraepithelial lymphocytes) [38][39]. Enteric neurons and gut immune cells communicate with each other in order to coordinate their actions against stressors [40]. Among the neuroendocrine compounds produced by the hypothalamic-pituitary-adrenal and sympathetic-adrenal medullary axes are corticosterone and catecholamines, which have the potential to influence immune regulation and phagocyte activity in a variety of immune cells [30]. Physical restrictions such as a low gastric pH and rapid transit in the small intestine play a crucial role in preventing pathogens from colonizing the gastrointestinal tract and causing inflammation [33]. In addition, pathogens must overcome the physical barriers imposed by the epithelium and intestinal microflora as well as the response of the host defense system in order to ultimately cause infection [41]. Cristofori et al. [42] showed that some non-pathogenic gut microbiota altered physiological cellular responses and the ability of an organism to fight infections by interacting with the host defense system and epithelium. Other potential advantages of probiotics are based on their significant impact on the intestinal environment. Epithelial and dendritic cells that constitute sentinel cells in the mucosa are found in lymphoid tissue connected to the intestinal tract. The binding of probiotic microbe-associated molecular patterns to Toll-like receptors on sentinel cells triggers NF-kB and MAP kinase pathways [42]. This activation does not only exert cytoprotective effects but also increases or inhibits the expression of genes controlling the inflammatory process by stimulating, signaling, and interpreting antimicrobial factors [43]. Additional advantages include enhanced epithelial barrier function, bacterial adhesion to the intestinal epithelium, and inhibition of microbial adhesion [33]. Probiotics are considered potential alternatives to antibiotics for improving immune health and growth performance in broilers. Cheng et al. [44] reported that dietary supplementation with Bacillus licheniformis enhanced T-cell immunity without impairing bird growth. It also directly impacted chemokine expression of genes and enhanced the production of pro- and anti-inflammatory cytokines in the mucosal surface, which had a profound effect on the immune system. Probiotics also influence immune function by affecting B-lymphocytes. Two bioactive secondary metabolites produced by probiotic bacteria, short-chain fatty acids, and bacteriocins, prevent infectious agents from growing and surviving [45]. Notably, several Lactobacillus strains producing lactic acid were found to be able to lower the pH level of their surroundings. Lie et al. [9] observed that Bacillus amyloliquefaciens initially reduced the stress caused by the immune response in lipopolysaccharide-challenged broiler chickens and increased plasma lysozyme activity and WBC count in 192-day-old males. Consequently, Bacillus amyloliquefaciens was able to restore impaired immune status and growth performance [46]. Yitbarek et al. [47] fed a mix of probiotics obtained from various strains of Bacillus subtilis, Lactobacillus acidophilus, and Lactobacillus casei and prebiotics (yeast-derived carbohydrates) to 300-day-old Lohman pullets. In this case, synbiotics enhanced the immune system and maintained homeostasis through an IL-10-specific response. Synbiotic supplementation resulted in the increased concentrations of IL-6, interferon-γ (IFN), and IL-4 in the ileum.
Hetab et al. [48] demonstrated that the production of antibodies against the Newcastle disease virus in layers was significantly enhanced by probiotic bacteria (Bacillus subtilis and Enterococcus faecium). Therefore, broiler chickens supplemented with B. subtilis showed higher levels of antibodies against Newcastle disease, infectious bronchitis, and bursal disease [49].

This entry is adapted from the peer-reviewed paper 10.3390/fermentation8120672


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