 The chemical cycling carried out by microbes occurs not only at a macroscale with planetary implications, but also internally, in our very own bodies. Symbiosis, the intimate “living together” of different organisms, is a key organizational motif of the microbial world, but it reaches far beyond microorganisms to encompass every (relatively) large living thing. Long recognized and conceptually debated, symbiosis is usually deemed to include a variety of relationship types: negative consequences for one of the organisms (parasitic), positive outcomes for both (mutualistic), and various mixes of beneficial and “non-harm, non-help” relationships (commensal) between microbes and their partners (Saffo 1992; Sapp 2004).