Although microbes influence the health and disease of all animals, we still understand little about mechanisms underlying microbial specificity for hosts and tissues or microbial contributions to host biology. To address these questions, my lab investigates a relatively simple tripartite association in which nematodes of the genus Steinernema parasitize insects by virtue of their mutualistic association with Xenorhabdus bacteria. The Steinernema-Xenorhabdus-insect symbiosis offers numerous advantages as a model to understand the specificity and processes of beneficial and pathogenic microbial colonization of host tissues. These include its experimental tractability: Each member of the tripartite association can be reared separately or together in varied combinations in the laboratory, at least one representative of each has a sequenced genome and transcriptome, and numerous genetic techniques have been developed for Xenorhabdus bacteria. Furthermore, investigations are simplified by the fact that Steinernema are colonized by a monoxenic culture of a specific species of Xenorhabdus bacteria, rather than a complex microbial consortium. Thus, the system can be probed to understand the molecular basis for specificity and the impacts of bacteria on animal host behavior, reproduction, immunity, and physiology. Finally, the Steinernema-Xenorhabdus mutualism is a conserved characteristic of both genera, facilitating evolutionary and comparative analyses of symbiosis mechanisms. The potential of the Steinernema-Xenorhabdus symbiosis to reveal fundamental molecular and cellular processes mediating symbiosis is inhibited by a lack of genetic and molecular tools to manipulate the nematode. Furthermore, although several C. elegans researchers have made advances in understanding several aspects of Steinernema molecular biology, much remains to be learned regarding Steinernema physiology, behavior, development, and immunity with respect to their symbiosis with Xenorhabdus.
Research in my lab focuses on understanding the molecular basis of bacterial symbiosis with animal hosts, and what distinguishes beneficial and pathogenic interactions. I will focus my talk on the molecular basis of species specificity between nematodes and bacteria. We have determined that X. nematophila bacterial association with its S. carpocapsae nematode host is tissue- and species-specific and requires the integral outer membrane protein NilB; Bacteria adhere to the anterior intestinal epithelial surface of adult and juvenile nematodes only when expressing nilB. The consequences of this surface colonization are as yet unknown, but possibilities include bacteria-host cell signaling that promotes pheromone production or immunity. The molecular function of NilB also is unknown. It is a beta-barrel protein with surface exposed loops both necessary for nematode interactions and a predicted small channel with an electronegative strip. These regions may be important for NilB binding to host cell surfaces or for transport of host-derived molecules. NilB is the only characterized member of a family of OMPs found exclusively in symbionts of animals (e.g., Neisseria, Haemophilus, Moraxella, Actinobacillus). Therefore, understanding NilB function in Xenorhabdus–Steinernema mutualism will provide insights into NilB homolog function in human associated microbes.
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