Doctor of Philosophy
Microbiology and Immunology
Dr. David Heinrichs
To microbes and humans alike, iron is both essential, and potentially toxic, where homeostatic concentrations must be stringently maintained. Within the iron-restricted host, the survival and proliferation of microbial invaders is contingent upon exploiting the host iron pool. Staphylococcus aureus is a formidable pathogen, whose success is partly attributable to its multiple, and often redundant mechanisms for acquiring iron. A decade of research in staphylococcal iron acquisition has identified several uptake mechanisms, including, but not limited to, the elaboration of high-affinity iron scavenging siderophores (staphyloferrins A (SA) and B (SB)) and the expression of an iron-regulated surface determinant (Isd) pathway for heme-iron acquisition. Factors influencing the expression or utility of these systems in vivo, however, have largely gone unaddressed and were the subject of this thesis.
Both SA and SB are citrate-based siderophores and, in Chapter 2, we endeavored to identify the enzymatic source of this staphyloferrin precursor molecule. We demonstrate that citrate derived from the TCA cycle citrate synthase, CitZ, is essential to the production of SA, whereas SB may obtain citrate from either CitZ, or the iron-regulated citrate synthase, SbnG. Activity of the TCA cycle is thus a key determinant in the elaboration of SA. Notably, the TCA cycle is downregulated during iron-restricted glycolytic growth, which we suggest has fundamental implications to the in vivo expression of SA. We propose that S. aureus requires two citrate synthases because SA and SB each fulfill unique, yet partially overlapping, roles in the metabolism and pathophysiology of the bacterium.
Although Isd has been extensively characterized in heme-iron acquisition, isd mutants often lack a growth-defective heme-dependent phenotype. In Chapter 3, we report that Isd functions as a biologically relevant, high-affinity heme acquisition system, with utility on physiological concentrations of hemoglobin. Isd function is masked at high heme concentrations, which we hypothesized to occur through low-affinity heme acquisition by a putative di-/tripeptide (DtpT) or oligopeptide permease (Opp). No Opp was found to function in heme acquisition, and while DtpT was found to have a reproducible yet minor role in this function, under the conditions tested, it was confirmed as a transporter of di-/tripeptides.
Sheldon, Jessica R., "Iron Acquisition Mechanisms and Their Role in Staphylococcus aureus Survival and Virulence" (2015). Electronic Thesis and Dissertation Repository. 3308.