We are investigating the contribution of sphingolipid production by the Bacteroidetes in the human gut as a fundamental service in human-microbiome interactions. The phylum Bacteroidetes is highly prevalent as members of the gut microbiome across animal species. In humans, representative general may differ between populations, but the phylum comprises 40-50% of the microbiome, on average. The Bacteroidetes stand out as one of only two known phyla (of >100) that have the capacity to produce sphingolipids. The source of sphingolipids for the human host is the diet and a small but important contribution from de novo production. Since humans carry sphingolipid-producin bacteria in their guts, we are investigating the importance of this endogenous source in human sphingolipid homeostasis in various body sites.