Proposed modes of action of livestock probiotics. Schematic diagram illustrating potential mechanisms, whereby oral administration of probiotics might promote beneficial effects by changing the composition of intestinal microbiota, altering intestinal barrier function, bile salts, and production of Th1 cytokines. Additionally, probiotics containing LAB may down-regulate the expression of pro-inflammatory cytokines and chemokines. Decrease in the translocation of bacteria may occur as a result of the ability of probiotics to tighten the mucosal barrier. Probiotics disallow colonization by pathogenic bacteria through competition for nutrients, immune system up-regulation, and production of antitoxins. These mechanisms include ① Competitive exclusion for binding sites, ② Adhesion to the GIT,③ Enhancement of the epithelial barrier, ④ Increase in digestion and absorption of nutrients, ⑤ Competing with pathogenic bacteria for nutrients in the gut, ⑥ Production of AM substances, ⑦ Alteration in gene expression in pathogenic microorganisms, ⑧ Bacterial antagonism, ⑨ Bioconversion and ⑩ Immunomodulation. Abbreviations: ↑, increased; ↓, decreased; Th1, Type 1 T helper; Th2, Type 2 T helper; IEC, intestinal epithelial cells; DC: dendritic cell.
①
Modification of the microbial population of the GIT: Probiotics might boost the population of beneficial microbes, such as
Lactobacillus and
Bifidobacterium, which subsequently restrict the growth of harmful bacteria by creating inhibitory chemicals and by competing for binding sites
[115,116][41][42]. ②
Adhesion to the GIT wall to prevent colonization by pathogenic microorganisms: The majority of enteric pathogens might colonize the intestinal epithelium and cause disease as a result
[117][43]. As a result,
Lactobacillus can adhere to the gut epithelium and compete with pathogens for adhesion receptors, such as glycoconjugates
[118][44]. The
Lactobacillus and
Bifidobacterium have hydrophobic surface layer proteins that assist the bacteria non-specifically by adhering to the animal cell surface
[119][45]. ③
Enhancement of the Epithelial Barrier: The experimental studies in model animal have shown that probiotics
P. acidilactici improve intestinal barrier function by reducing the permeability of the intestinal epithelium translocation of enterotoxigenic
E. coli to mesenteric lymph nodes in post-weaning piglets as compared to the control group after ETEC challenge
[120][46].
OurThe current findings suggest that the
L. jensenii TL2937 reduce the intracellular Ca
2+ flux in DSS-challenged PIE cells, increasing the tightness of the tight junction
[121][47].
④
Increase in digestion and absorption of nutrients: In this case, the spore-forming bacteria enhance the production of extracellular enzymes, which facilitate nutrient digestion
[122,123][48][49]. ⑤
Competing with pathogenic bacteria for nutrients in the gut: Probiotic bacteria might compete with pathogenic bacteria for nutrients and absorption sites by rapidly utilizing energy sources, potentially shortening the log phase of bacterial development
[116][42]. ⑥
Production of antimicrobial substances: Several probiotic bacteria, particularly those that produce lactic and acetic acids, have the ability to suppress harmful microorganisms
[124,125][50][51]. ⑦
Alteration in gene expression in pathogenic microorganisms: Probiotics might influence pathogenic bacteria’s quorum sensing, hence altering their pathogenicity. Fermentation products from
L. acidophilus La-5 significantly suppressed the extracellular production of a chemical signal (autoinducer-2) by human enterohaemorrhagic
E. coli serotype O157:H7, leading to inhibition of the virulent gene (LEE—locus of enterocyte effacement) expression in vitro
[126][52]. ⑧
Bacterial antagonism: Probiotic microorganisms, once established in the gut, may produce organic acids, hydrogen peroxide, lactoferrin, and bacteriocin, which may exhibit either bactericidal or bacteriostatic properties
[127][53].
⑨
Bactericidal activity/
Bioconversion:
Lactobacillus convert lactose to lactic acid, lowering the pH to a point where pathogenic bacteria cannot survive. Furthermore, living yeasts compete with lactic acid-producing bacteria to digest sugars obtained from starch breakdown, thereby stabilizing rumen pH and minimizing the danger of acidosis
[128,129,130][54][55][56]. ⑩
Immunomodulation:
Our sIt
udy has is shown that probiotic LAB with immunoregulatory functions can beneficially modulate the immune response in the gut by modulating the functions of PIE cells
[12,54,56][12][57][58]. In addition, probiotic LAB have proven to be capable of acting as immune modulators by enhancing macrophage activity
[54][57], increasing local antibody levels, inducing the production of anti-inflammation cytokines (interleukin (IL)- 10, interferon (IFN)-γ, β, IL-1β, TGF-β), reducing IL-4, IL-6, IL-8, MCP-1, and activating killer cells
[11,32,54][11][59][57].
Immunomodulation properties appear to be strain dependent, which means that dissimilar probiotics might have parallel mechanisms of action, whereas a single strain may have multiple mechanisms of action. Quite a lot of probiotic strains, for example, have comparable impact on the microbial community of gastrointestinal tract, although the mechanisms of action of certain probiotics are mostly unknown. The exact mode of action of probiotics is not well understood in the majority of studies on their impact on performance. Therefore, the mechanisms must be explored on a case-by-case basis because closely interrelated probiotics appear to have diverse ways of action. Probiotic effects are a result of the interaction between the host and the probiotic microorganism. As a result, more research into the host–microbe interaction could shed light on the probiotic mode of action. Rapid improvements in molecular techniques and genome sequencing for microbial ecology research will substantially aid our understanding of probiotic mechanisms of action.