UC San Diego

Nematodes and their naturally associated pathogens are useful systems for dissecting host-pathogen interactions in vivo. Understanding the mechanisms of pathogenesis and innate immunity from these models could draw parallels to relevant human diseases and open new potentials for therapeutic interventions. This dissertation focuses on a new system, the free-living soil nematode Oscheius tipulae and its intracellular bacterial pathogen Bordetella atropi. To understand what nematodes, representative by Caenorhabditis elegans, can offer in terms of gaining relevant insights into the biology of immunity, the first chapter will review current knowledge on nematode immune responses against a wide variety of pathogens and how they are functionally and conceptually similar to those in vertebrate immunity. Chapter two will then describe our discovery that B. atropi employs filamentation – a morphological change from the normal coccobacillus to filamentous shape – for spreading from the initially infected cell to neighboring cells. We showed that this filamentation process is under control of a metabolic pathway highly conserved among bacterial species, which has been shown to regulate bacterial cell sizes under rich conditions. From the current available data, we proposed a model in which B. atropi, upon entering host cell, detects the rich environment of host cytoplasm and initiates filamentation to quickly spread to other cells, maximizing host cell space and resource utilization. Furthermore, we posited that spreading by means of filamentation represents a new paradigm of intracellular dissemination compared to that of more well-studied mammalian intracellular bacteria, including Rickettsia spp., Shigella spp., Burkholderia spp., and Listeria monocytogenes, where the pathogens often hijack host actin polymerization to form actin tails for the same purpose. Chapter three will present the mechanism that B. atropi uses to invade intestinal cells, helping it establish a replicative niche prior to filamentation. We found that the bacteria require a type III secretion system, a multi-subunit machinery that delivers proteins called effectors into host cells, for internalization. This invasion process results in host cells forming structures arching over incoming bacteria that appear morphologically similar to a form of endocytosis known as macropinocytosis. Soon after uptake, the bacteria quickly escape from endocytic vacuoles to gain access into host cytoplasm before filamenting. Altogether, we characterized the infection process of an intracellular bacterial pathogen in vivo from invasion to intracellular dissemination and uncovered a new mechanism of cell-to-cell spreading, providing a useful model for studying intracellular bacterial pathogenesis and host innate immunity.