A direct or indirect barrier effect, maturation of the immune system, the intestinal microbiota protects its host against pathogens in many ways.
The intestines are a preferred portal for invasion by pathogenic organisms and toxic molecules, meaning the intestinal microbiota is our first line of defense against these attacks. This gut flora acts in many ways.
Via a competitive exclusion mechanism, commensal bacteria passively protect the body against infection by other strains, by competing with them for adhesion sites and the nutrients essential to their survival. They also act more directly by producing metabolites that are harmful to their competitors, such as antibacterial peptides.
Reinforcement of natural defenses
Commensal bacteria also protect us by reinforcing the intestinal barrier. They stimulate the production of mucus and defensive molecules, such as IgA antibodies, and activate intestinal epithelial cell replacement and the formation of tight junctions between them, maintaining the impermeability of the physical barrier1,2,3 .
Effect on the intestinal immune system
Lastly, during the first years of our lives, the intestinal microbiota participates in immune-system maturation. For example, the gut flora stimulates the development of T-helper 17 (Th17) cells4 , through a process based on the secretion of molecules that can cross the intestinal epithelium and interact with specific cell surface receptors. This action is not limited to immune defense around the intestines; the microbiota seems to be able to influence responses to all kinds of threats (for example, respiratory infections)5.
1. Jakobsson HE, Rodriguez-Pineiro AM, Schutte A, et al. The composition of the gut microbiota shapes the colon mucus barrier. EMBO Rep 2015;16:164-77.
2. Seth A, Yan F, Polk DB, Rao RK. Probiotics ameliorate the hydrogen peroxide-induced epithelial barrier disruption by a PKC- and MAP kinase-dependent mechanism. Am J Physiol Gastrointest Liver Physiol 2008;294:G1060-1069.
3. Reikvam DH, Erofeev A, Sandvik A, et al. Depletion of murine intestinal microbiota: effects on gut mucosa and epithelial gene expression. PLoS One 2011;6:e17996.
4. Caballero S, Pamer EG. Microbiota-mediated inflammation and antimicrobial defense in the intestine. Annu Rev Immunol. 2015;33:227–56. 10.
5. Denny JE, Powell WL and Schmidt NW (2016) Local and Long-Distance Calling: Conversations between the Gut Microbiota and Intra and Extra-Gastrointestinal Tract Infections. Front. Cell. Infect. Microbiol. Année;6:41.