PROJECT SUMMARY Parasitic nematodes colonize over a billion people worldwide and are associated with a range of maladies. Among these parasites, the whipworm Trichuris trichiura is particularly difficult to manage with existing medication that target adult worms due to drug resistance and frequency of reinfection. Following ingestion by the host, Trichuris eggs hatch in the gastrointestinal tract where they mature into adulthood and produce eggs, which are then deposited back into the environment in feces to perpetuate the cycle. As such, the entire time the parasite inhabits the host is spent in the presence of a diverse community of bacteria that are part of the gut microbiota. We and others have shown that colonization by Trichuris trichiura and the model parasite Trichuris muris restructures intestinal bacterial communities in humans and mice, respectively. Consistent with interactions between parasitic nematodes and the gut microbiota, bacteria are required for T. muris to complete its life cycle in mice due to a role for bacteria in promoting egg hatching. The details of how the gut microbiota mediate egg hatching and the reproductive fitness of Trichuris remain obscure. In preliminary results, we identified bacteria that differentially affect T. muris egg hatching and established a novel C. elegans screen to identify bacterial gene products that have a conserved role in the reproductive fitness of free-living and parasitic nematodes. The main objective of this proposal is to identify mechanisms by which bacteria mediate Trichuris egg hatching and affect subsequent colonization of the host. First, based on our findings that T. muris and C. elegans share a requirement for an arginine-dependent byproduct of E. coli, we will identify this metabolite and determine how it impacts host reproduction using the two nematode models. We will specifically test the hypothesis that polyamines are the key arginine-dependent factors in this system. Then, we will delve deeper into how bacteria-egg interactions mediate hatching by determining the role of chitinase produced by the parasite and compare bacterial species that promote versus impede hatching. Lastly, we will determine whether bacteria displaying superior egg-hatching activity enhance T. muris infection of mice, and using a new assay we developed for T. trichiura, extend our observations to the human parasite. We believe these innovative approaches will improve our understanding of how parasites have adapted to their host environment, and ultimately reveal vulnerabilities in the Trichuris life cycle that can serve as therapeutic targets.