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Siderophore-mediated Competition for Iron Between Humans and Pathogenic Bacteria



Siderophore-mediated Competition for Iron Between Humans and Pathogenic Bacteria


Allyson Kawile Sia

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Kenneth N. Raymond, Chair

Iron is the most abundant and essential transition metal used by many living organisms. To understand the means by which organisms acquire, transport and use iron expands the understanding we humans have of life. Organisms evolved multiple strategies to solubilize iron hydroxides from the environment and to increase iron bioavailability. Most bacteria, fungi and plants solubilize iron from the environment by synthesis and secretion of siderophores, or small molecules that have a high affinity for ferric iron. Alternative iron acquisition strategies are often employed simultaneously which include piracy of iron stores from other organisms and exploitative uptake of nonnative siderophores and hemes.

Generally this work is meant to provide detailed information for mechanisms and strategies involved in iron competition between pathogenic bacteria and human hosts. Detailed structural and kinetic investigations of iron acquisition pathways in the Bacillus cereus group of bacteria are discussed in Chapters 1-4. The B. cereus group of bacteria includes threatening pathogens such as the anthrax pathogen (B. anthracis) and enterotoxic bacteria (B. cereus). Iron complexes of molecules as small as citrate and as large as a polymeric bacterial capsule are probed for structural and kinetic data related to iron acquisition and transport. Studies of the iron binding ability and subsequent transport of synthetic analogs of the B. anthracis stealth siderophore petrobactin are also included. Taken together, these studies shed light on multiple mechanisms and structure-function relationships necessary for the pathogenic bacteria to obtain an adequate iron supply and regulate virulence.

Bacillus bacteria can infect humans and compete with the host iron supply. The consequences of iron piracy by Bacillus anthracis are especially likely to be fatal. Crucial to the human defense against bacterial infections is the immune protein siderocalin. Siderocalin is antibacterial and limits bacterial growth by intercepting bacterial siderophores. Chapter 5 highlights siderocalin and its recently established alternative role as an iron transport protein. For siderocalin to transport iron, an endogenous siderophore equivalent is required. A putative endogenous, or `mammalian' siderophore, is evaluated by solution thermodynamic analyses, inorganic spectroscopy and biochemical assays. The multipurpose role of siderocalin could have important clinical impact, and it is important that the foundation of these studies are clear and consistent at the chemical level. The elucidated roles of siderocalin in humans convey key iron-restricting strategies by humans to compete with invading pathogens for iron. The collection of chapters in this work portrays the competition for iron as dynamic, aggressive and evolving by both humans and invading bacterial pathogens and is meant to provide a snapshot of the competition in current times.

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