Bacterial Factors Involved in the Xenorhabdus nematophila-Manduca sexta Pathogen-Host Interaction

Xenorhabdus nematophila is a bacterial insect pathogen that can suppress innate immune responses and kill insects. This thesis examines the contribution of specific bacterial factors in modulation of and resistance to insect immunity and the underlying mechanisms of X. nematophila pathogenesis. During infection X. nematophila overcomes or prevents induction of insect humoral constitutive defenses, such as the pro-phenol oxidase (proPO) serine protease cascade necessary for the production of toxic byproducts and melanin that entraps pathogens. Two X. nematophila factors, ecotin and rhabdopeptides, contribute to inhibiting Manduca sexta proPO serine protease cascade activation. The X. nematophila serine protease inhibitor ecotin can reduce activation of the proPO system. However, its specific M. sexta target remains unknown. Furthermore, rhabdopeptides, which are non-ribosomal peptides, also reduce the expression of the pattern recognition receptor hemolin, which is induced through the Toll immune signaling pathway. Rhabdopeptide-mediated inhibition is caused by its ability to interfere with the activation of serine protease hemolymph proteinase 6, part of the humoral immune pathway induction cascade. In addition to suppressing immunity, X. nematophila can persist in the presence of antimicrobial peptides (AMPs), an insect pathogen-induced response, in part due to the two-component system regulator PhoPQ and the PhoPQ regulated gene nlpC. nlpC is part of a PhoPQ-regulated locus predicted to encode lipopolysaccharide (LPS) modification machinery, suggesting LPS modifications play a role in X. nematophila persistence in the presence of AMPs. Finally, the outcome of the interaction between X. nematophila and M. sexta is determined by Lrp protein levels. Low levels of Lrp are associated with a virulent phenotype, while cells that lack Lrp or express high Lrp levels are attenuated. Attenuation of an lrp null mutant is due to its lack of lrp-dependent toxic activities. In contrast, virulence attenuation of a high-lrp strain is due to hypersensitivity to host immunity and the inability to produce or secrete insecticidal factors. Identification of these factors and virulence mechanisms provides new avenues for controlling pathogens and insects that are detrimental to human health and agriculture.