Purine Acquisition and Biosynthesis in Helicobacter Pylori

Helicobacter pylori is a gram negative bacterium that chronically colonizes the gastric mucosa of roughly half the world's population. Over millennia, this microbe has evolved within humans and has become incapable of freely living in the environment. As a result, this human-dependant pathogen must rely heavily on its host for survival. Investigations into the genome of this ardent pathogen have shown that H. pylori exhibits a remarkable level of genomic efficiency, as evidenced by an apparent absence of phage, a lack of metabolic redundancy, and a reduced ability to utilize a number of various substrates. This apparent streamlining of the H. pylori genome suggests that this bacterium has adapted to the unique environment within its human host over the course of evolutionary time. An example of this streamlining can be found in the examination of the purine nucleotide biosynthesis pathway. Our analysis of the purine nucleotide biosynthetic pathway in H. pylori has led us to the conclusion that this human-dependant pathogen has lost the ability to generate purine nucleotides de novo. We show that human serum, while containing some purines, does not appear to be sufficient to meet H. pylori's purine requirements in vitro, and that a richer purine source available to the bacterium in vivo may be extracellular DNA released by gastric epithelial cells. We also investigate two putative H. pylori purine transporters (NupC and PbuG) and their effects on growth under a variety of conditions, as well as explore the effect outer membrane permeability plays in purine acquisition. While the majority of studies to date have explored only the adherence properties of this family of outer membrane proteins, we have found that by deleting various porin genes in H. pylori, bacterial growth is severely reduced under nutrient poor conditions. We conclude that H. pylori is a purine auxotroph iv that fully relies on its host to provide the purines that are essential to its survival. We also conclude that H. pylori outer membrane proteins that were previously characterized as essential to colonization due to their adherence properties actually affect the ability of this bacterial pathogen to acquire nutrients. v Dedication For my parents, Michael and Barbara Liechti, who have instilled in me both a love for science and a borderline masochistic work ethic. You have provided me with every advantage regarding my education, as well as the emotional support that has helped me through my graduate career. I love you both very much, and thank you for everything that you have given and sacrificed for me. This dissertation is for you.

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