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Date of Award
Doctor of Philosophy (PhD)
Transition metals constitute an important part of the host-pathogen interface. Iron is an essential nutrient that functions as a cofactor for numerous bacterial and host proteins, as either a ligand for oxygen in carrier proteins or an enzyme catalytic site due to its natural redox properties. As part of the innate immune response, infected hosts sequester iron from pathogens to limit their growth, a phenomenon known as nutritional immunity. On the other hand, copper ions are deployed at infection sites as a potent antimicrobial agent to kill bacteria. The ability to survive within multiple, often harsh, microenvironments is fundamental to not only the colonization of a host by bacteria, but also for disease establishment and spread. Bacteria, therefore, express multiple virulence factors that, although not universally necessary for growth and replication, are critical for survival within these harsh microenvironments.
Prominent among these virulence factors are siderophore systems, which make and secrete small secondary metabolites to bind extracellular iron with extremely high affinities and form stable iron-siderophore complexes. These complexes are then imported via dedicated machinery to deliver iron as a source of nutrition. Siderophores represent a significant means by which bacteria acquire host-sequestered iron. Curiously, some bacterial species have acquired the ability to express multiple siderophore systems simultaneously. In uropathogenic E. coli (UPEC), the leading cause of human urinary tract infections (UTI), as many as three siderophore systems can be expressed by a single bacterium in addition to the universally conserved enterobactin system. Given the metabolic cost of siderophore biosynthesis, it would appear maladaptive to express multiple siderophores for the seemingly redundant function of iron acquisition. Previous studies determined that secondary, non-enterobactin siderophores perform either niche-specific or non-iron functions that confer a survival advantage to expressors. Perhaps, the most remarkable of such secondary siderophores is yersiniabactin (Ybt).
Ybt is a natural biosynthetic product of the Yersinia High Pathogenicity Island (HPI). It has been traditionally characterized as an iron acquisition system due to its ability to bind and deliver iron to bacteria for nutrition. Recently, it was determined that Ybt can bind other transitional metal ions in addition to iron. Prominent among these is copper, which acts as both nutrient and toxin to bacteria. By binding copper, Ybt protects bacteria against copper toxicity, while retaining the ability to deliver the metal to bacteria for nutrition, a phenomenon termed nutritional passivation. Given the diversity of metal ions that interact with Ybt, we hypothesized that Ybt is regulated differently in response to the different metal ions. In this dissertation, I will present work that shows that Ybt is repressed in the presence of iron ions and upregulated in the presence of copper ions. While repression by iron is consistent with Fur-mediated repression of siderophore systems, upregulation by copper ions is a novel finding among siderophores. Remarkably, copper-associated upregulation was independent of Fur and canonical E. coli copper sensors but required Ybt biosynthesis and YbtA activity. We also determined that there is crosstalk between the Ybt system, which can import copper, and the Cue system, which is recognized as the central component of E. coli copper detoxification. Finally, our work showed that the Ybt system is actively expressed during experimental mouse bladder infections with a model UPEC, UTI89. As a body, this work advances our understanding of in vitro and in vivo Ybt expression and brings us closer to being able to exploit the Ybt system as a novel target for the development of anti-virulence therapies against UTI.
Chair and Committee
Jeffrey P. Henderson
Daniel E. Goldberg, Joseph P. Vogel, Christina L. Stallings, Eric A. Galburt,
Katumba, George Lwanga, "Copper-mediated Regulation of a Traditional Iron Uptake System in Uropathogenic Escherichia coli." (2021). Arts & Sciences Electronic Theses and Dissertations. 2505.
Available for download on Thursday, June 08, 2023