Insights into Steinernema carpocapsae-Xenorhabdus nematophila specificity and the role of adult nematode colonization

The gut microbiota can provide the host with several benefits such as the production of secondary metabolites, essential amino acids, as well as the breakdown of food or protection from pathogens. The host in turn provides the microbiota with shelter and nutrients. Given the benefits that the host and microbiota receive from their association, humans and the microbiota have evolved mechanisms of selection and specificity to assure the proper symbionts colonize a host with high fidelity. Chapter two will explore mechanisms by which Xenorhabdus nematophila nematode intestinal localization (nil) genes interact with and adhere to the nematode intestinal cell surface at the anterior intestinal cecum (AIC). The AIC expresses a mucus lectin wheat germ agglutinin (WGA) is able to bind to, suggesting that the mucus contains N-acetylglucosamine and N-acetylmuramic acid. This mucus might be serving as mechanism of bacterial adherence or nutritional selection at either the adult of infective juvenile (IJ) life stages of the nematode. Along with providing the above benefits to each other, host and gut microbiota can impact and influence each other the bi-directional line of checmical communication called the gut-brain axis. The use of model systems to study host-microbiota interactions can provide insights, but such studies are complicated by the diversity and number of cells within the gut, making pinpointing the bacterial species responsible for a specific impact on the host difficult. Chapter three will demonstrate that S. carpocapsae females reared on lawns of X. nematophila rpoS::kan have significantly reduced egg laying compared to females reared on wild type X. nematophila lawns, and that egg laying in the females grown on mutant lawns will increase their egg laying with the addition of exogenous dopamine, suggesting that the X. nematophila mutants might be impacting the levels of neurotransmitters found in the nematode. This work attempts not only to better describe the ways that S. carpocapsae nematodes select for X. nematophila symbionts through the use of WGA-reactive material expressed at the site of colonization, but also how that selection as the plays a role in controlling S. carpocapsae female reproductive behaviors possibly through the gut-brain axis.