Introduction
Bacteria play, for better or worse, a fundamental role in human life. As multidrug resistant bacterial infections are increasing in incidence and mortality, we often consider only the negative impact of bacteria on human life and forget the positive side of the “bacterial coin” – the microbiota. Commensal microbes are critical components that contribute to maintain and promote our health in a complex variety of ways. The gut microbiota is now regarded as an organ with roles in shaping our immunity, host defense and intestinal maturation and function (Moron et al. 2019).
Intestinal Epithelium and Commensal Flora
Intestinal mucosa is composed of epithelial cells closely joined together by tight junctions acting as a barrier to restrict substance passage between cells. Epithelial cells are anchored to a thin layer of connective tissue that hosts immune cells underneath which lies the muscolaris mucosae. Other mechanisms of intestinal defense include gut associated lymphatic tissue (GALT) and mesenteric lymph nodes, mucus production and commensal bacteria; together they compose the intestinal barrier (Assimakopoulos et al. 2018).
Microbiota functions are executed mainly through its composition: commensal microorganisms compete with opportunistic pathogens for adhesion sites and nutrients creating a first line of defense against bacterial translocation (Wang et al. 2019). Additionally, commensal microbes shape the mucosal immune system by regulating T cells expansion and differentiation, dendritic and macrophage activation and B cells produced IgA (Yamashiro 2017). T cells dependent IgA are induced in response to specific microbes in the gut and protect against lethal sepsis following intestinal barrier disruption. Their concentrations depend on a rich and diverse microbiota; in particular Proteobacteria resulted in increased IgA concentrations in murine models (Wilmore et al. 2018).