Shared metabolism between a bacterial and fungal species that reside in the human gut.

The fungal species Candida albicans and the bacterium Enterococcus faecalis are members of the human gut microbiome. To explore the range of interactions between these two species, we utilized dual RNA-sequencing to transcriptionally profile both C. albicans and E. faecalis during coculture (compared with monoculture controls) under two conditions: 1) an in vitro setting that mimics certain features of the gut environment and 2) a gnotobiotic mouse gut model. RNA-seq analysis revealed a large number of gene expression changes induced by one species in the presence of the other. More specifically, both species highly upregulate citrate-metabolizing genes during coculture: C. albicans upregulates CIT1 (citrate synthase) which produces citrate, while E. faecalis upregulates its cit operon, which breaks down citrate. In vitro analysis showed directly that citrate cross-feeding (production of citrate by C. albicans and breakdown by E. faecalis) enhances growth of E. faecalis. A main byproduct of citrate metabolism in E. faecalis is formate, a short chain fatty acid toxic to fungi. Our RNA profiling revealed that C. albicans upregulates three formate dehydrogenases (FDHs) during coculture; we show that the FDH genes confer a growth advantage to C. albicans when E. faecalis (or simply formate) is present. These findings reveal a metabolically driven cycle between C. albicans and E. faecalis in the mouse gut and in vitro, where cross-feeding of citrate and detoxification of formate facilitates the growth of both species when they are cultured together.