Fig. 1

Important mechanisms of commensals involved in colonization resistance. The commensals in the gut play a key role in preventing the expansion of potential pathogens, thereby, contributing to colonization resistance. The most important mechanisms of commensals involved in colonization resistance comprise: (A) Commensals provide a direct barrier to colonization by pathogens through competing for space and nutrients; (B) Commensals continuously stimulate pathogen recognition receptors (PRR), like Toll-like receptors (TLR), on intestinal epithelial cells (IEC) to secrete protective mucins (MUC) and antimicrobial peptides (AMP); (C) Commensals contribute to adaptive immunity by stimulating secretion of secretory immunoglobulin A (IgA) which provides protection against pathogens by crosslinking of pathogenic bacteria and neutralizing bacterial toxins; (D) Commensals generate microbial fermentation products, such as butyrate, which has trophic effects on the intestinal mucosa, thereby, enhancing the intestinal barrier