Initiation and maintenance of Steinernema carpocapsae nematode colonization by Xenorhabdus nematophila bacteria

Non-pathogenic (benign) microbe-host associations are ubiquitous in nature, yet the mechanisms by which benign microbes interact with their hosts have been understudied relative to pathogenic associations. The investigations documented in this thesis focus on understanding the mechanisms by which one benign microbial species, Xenorhabdus nematophila, interacts mutualistically with an invertebrate host, the entomopathogenic nematode Steinernema carpocapsae. These investigations contribute to the development of this model of benign microbe-host association with the goal of increasing the biological understanding of these types of relationships. X. nematophila lives within the intestine of S. carpocapsae and enhances the fitness of this nematode as an insect parasite. Other bacteria, including related Xenorhabdus spp. are excluded from colonizing the S. carpocapsae intestine, suggesting the evolution of a cognate relationship between these two species. The findings presented show that upon colonizing the intestinal tract of S. carpocapsae (initiation), X. nematophila grows within this host niche, apparently utilizing nutrients present within the host (in vivo) to assist in its own proliferation. To understand bacterial nutritional requirements during in vivo growth, various X. nematophila metabolic mutants were tested for their ability to colonize S. carpocapsae . Some of these mutants exhibit observable defects in host colonization while others do not. Analysis of aberrant colonization by colonization-deficient mutants has provided new insights into the normal colonization process, including the identification of a sub-cellular location in the nematode intestine that appears to be a focal point for colonization initiation. These experiments contribute to the development of this model system as a paradigm for in vivo growth and nutrient harvesting by benign microbes.