The salmonid gastrointestinal tract microbial community is largely dominated by Mycoplasma sp., however, the biological advantage that the host draws from this association is unknown. Here we report that both the PC and intestinal microbial communities of adult farmed rainbow trout are dominated by Mycoplasma sp. and that intestinal bacteria are able to de novo synthesize unique lipidic compounds that contribute to the host lipid metabolism. These findings are highly significant in the context of dysbiotic states in which trout lipidic sources and energy states may depend on the community composition (diversity and abundance) of intestinal bacteria