ABSTRACT
Animals exist in symbiosis with numerous bacteria, most of which are in the gut, and depend on them for health and fitness. Gut microbes closely interact with epithelial and local immune cells. In addition, through secreted metabolites or by activating endocrine signaling, microbes also affect remote tissues, which include in vertebrates the liver, bone, and even the brain. While communication is thought to be bidirectional, relatively little is known about the effects of host extraintestinal tissues on the gut microbiome. In this chapter we introduce the worm Caenorhabditis elegans as a new model for studying host–microbiome interactions, focusing on recent findings revealing extraintestinal contributions of TGFβ signaling to determining gut commensal abundance and function. We consider these findings in the broad context of extraintestinal control of the gut microbiome, and further consider extraintestinal control in the even broader context of cell nonautonomous regulation of homeostasis. Last, we explore an extensively characterized binary symbiosis—the interactions between the bobtail squid and its Vibrio symbionts, to gain insight into the molecular language that might be shared between different systems of host–microbe interactions.
