Bile acids in immunity: Bidirectional mediators between the host and the microbiota.

Host-microbiota interactions are bidirectional. On one hand, ecological pressures exerted by the host shape the composition and function of the microbiota. On the other, resident microbes trigger multiple pathways that influence the immunity of the host. Bile acids participate in both parts of this interplay. As host-derived compounds, they display bacteriostatic properties and affect the survival and growth of the members of the microbial community. As microbiota-modified metabolites, they further influence the microbiota composition and, in parallel, modulate the immunity of the host. Here, we provide a comprehensive overview of the mechanisms behind this unique dialogue and discuss how we can harness bile acids to treat intestinal inflammation.

Bypassing the Gut-Lung Axis via Microbial Metabolites: Implications for Chronic Respiratory Diseases.

The gut microbiome engages in constant interactions with the immune system, laying down the fundamentals of what we perceive as health or disease. The gut microbiota acts locally in the intestines and distally in other organs, such as the lungs. This influence (termed "the gut-lung axis") constitutes the basis for harnessing the microbiome to prevent or treat chronic respiratory diseases. Within this context, two approaches gained the most attention: the diet interventions (which shape the microbiome) and the probiotics (which exert beneficial effects directly on the host). Microbial products, which constitute a means of communication along the gut-lung axis, are only now emerging as a new class of potential therapeutics. Here, we provide a comprehensive overview of microbial products active in the airways, describe the immunological mechanisms they trigger, and discuss their clinical advantages and pitfalls.