A delicate balance: maintaining mutualism to prevent disease

Abstract

The intestinal microbial ecosystem is complex, and few of the principles that contribute to homeostasis in health are well understood. Pham et al. (2014) show that a network including the epithelial interleukin-22 receptor protects against infection with the opportunistic pathogen Enterococcus faecalis through promotion of host-microbiota mutualism.

Figure 1. Driving Forces in the Balance between Gastrointestinal Tract Homeostasis and Dysbiosis

On the left, healthy GI tract homeostasis is characterized by a large diversity of microbes, an intact mucin layer, epithelial cells decorated with fucosylated glycans, and patrolling immune cells that produce antibodies, cytokines such as IL-22, and other antimicrobial peptides, all of which help regulate the microbial community. Disruptive forces such as changes in diet, infection with a virulent microbe, inflammation, and/or antibiotic use can shift the community to a state of dysbiosis, shown on the right. In this state, the microbial community is thrown off balance, and the host is prone to systemic infection with enterococci. Administering probiotic bacteria, or simply probiotic metabolites such as 2′-fucosyllactose, can help restore microbial homeostasis.