Waqas Ishaq, Praveen Reddy Kasu, Bishnu Prasad Bhattarai, Ramesh K Selvaraj
Probiotics have profound effects on the host's physiological functions and gut mucosal immunity. The in ovo and early post-hatch probiotic supplementation provides a better opportunity for commensal bacteria to reside in the avian gut and minimize the invasion of pathogenic microbes. Probiotics colonize the gut epithelial cells through adhesion structures and mucus surface-binding proteins. The successfully colonized probiotic bacteria use direct and indirect mechanisms to resist the invasion of other bacteria to develop colonization resistance. Probiotics produce the secondary metabolites, such as short-chain fatty acids, hydrogen peroxide, enzymes, and bacteriocins, that inhibit the proliferation of other resident gut microbes. Probiotics inhibit the quorum-sensing activity through disrupting the signaling molecules' secretion, virulence factor gene expression, and biofilm formation. Probiotics improve the intestinal barrier functions through higher tight junction protein gene expression, which limits the entry of toxins, antigens, and bacteria into the systemic circulation. Probiotics modulate the immune response through altering the expression of toll-like receptors, which inhibits the downstream proinflammatory signaling pathways. Probiotics activate the polarization of T-regulatory cells and inhibit Th17 cells, which results in the production of anti-inflammatory cytokines and regulates the population of CD8+ cytotoxic and effector T cells. Probiotics also enhance the B cell immunity through increasing the secretory IgA levels in the gut lumen that neutralize the pathogenic bacteria. These mechanisms act simultaneously and reinforce each other. Engineered probiotics extend these functions to targeted delivery of antimicrobial peptides, virulence factor-targeting nanobodies, and vaccine antigens. The reported outcomes of probiotics are heterogeneous among poultry studies. The efficacy of a probiotic strain depends on the supplemented probiotic strain, dose rate, diet, bird type, and pathogen pressure. This review aims to provide insights into intestinal defense mechanisms, gut microbiota colonization and contributions, and mechanisms of action of probiotics.