Sterol-degrading actinobacteria function as an unexpected sterol source for sterol-auxotrophic Caenorhabditis nematodes

During evolution the nematode Caenorhabditis elegans has lost the ability to synthesize steroids de novo, making its survival dependent on exogenous sterols for reproduction and life stage development. Strikingly, C. elegans feeds almost exclusively on bacteria, which typically do not synthesize steroids, raising the question of how C. elegans acquires sterols in nature. We recently found that sterol-degrading Rhodococcus and Mycobacterium bacteria accumulate sterols in intracellular lipid droplets under stress conditions such as nitrogen limitation and hypothesized that these bacteria may constitute an important sterol source for C. elegans in oligotrophic habitats.

To test this hypothesis, we compared C. elegans 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 or with sterol-free bacteria when cholesterol was supplied in the growth medium. To differentiate bacteria- and medium-derived sterol uptake 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. In contrast, medium-derived cholesterol localized primarily in the nerve ring and sensory dendrites but was almost absent from the intestine, indicating differences in sterol uptake and transport. Feeding assays with C. elegans sterol uptake mutants showed that the intestinal cholesterol transporter CHUP-1 was required for uptake of bacteria-derived cholesterol into the worm body, while deletion of the sphingomyelin synthase SMS-5, which is involved in cholesterol absorption in the pharynx, had no significant effect. Proteomic analyses are currently underway to further investigate different physiological responses in C. elegans to bacteria- and medium-derived sterols.

Our results show that the sterol-auxotrophic C. elegans can use intracellular bacterial sterol storage compounds to maintain its reproductive cycle, suggesting that sterol-degrading actinobacteria represent a natural and previously unstudied sterol source for bacterivorous animals in oligotrophic habitats. This opens a new field of research into sterol-dependent regulation and developmental processes in C. elegans