Sterol-accumulating bacteria function as a cholesterol source for sterol-auxotrophic bacterivorous nematodes.

During evolution, some Ecdysozoa such as insects, nematodes, and crustaceans have lost the ability to synthesize steroids de novo, making them dependent on exogenous sterols. In Caenorhabditis elegans, dietary sterols are primarily required as hormone precursors to regulate reproduction and life stage development. Strikingly, C. elegans are filter-feeding bacterivores relying on bacteria as their main nutrient source. Since most bacteria do not synthesize steroids, it remains an open question how these nematodes acquire sterols to maintain their life cycle. We recently found that sterol-degrading Actinobacteria from the Rhodococcus and Mycobacterium genera accumulate significant amounts of sterols in intracellular lipid droplets under stress conditions such as nitrogen limitation. We hypothesized that these sterol-accumulating bacteria may constitute an important sterol source for C. elegans in oligotrophic habitats.

To test this hypothesis, we measured C. elegans reproduction rates by comparing brood sizes with different sterol-accumulating and sterol-free bacteria as food sources on sterol-free medium. While C. elegans was not able to produce offspring with sterol-free E. coli, M. smegmatis, or M. aurum, around 250 offspring were produced per worm when M. smegmatis and M. aurum had accumulated intracellular sterols. Similar numbers were produced with sterol-free bacteria on cholesterol-supplemented medium. To track bacteria- and medium-derived sterols in C. elegans, we performed feeding assays with fluorescently labelled cholesterol. Confocal microscopy showed that bacteria-derived cholesterol was taken up via the pharynx into the gut and across the gut wall of C. elegans. In contrast, medium-derived cholesterol localized primarily in the nerve ring and sensory dendrites but was almost absent from the pharynx and the intestine. Experiments with C. elegans sterol uptake mutants are currently underway.

Our results show that C. elegans can use intracellular bacterial sterol storage compounds to maintain its reproductive cycle and suggest that different uptake mechanisms for sterols derived from bacterial lipid droplets and from the medium exist in C. elegans.