Bacterial pathogens deploy sophisticated strategies to acquire vital nutrients from the host during infection. Staphylococcus aureus is a considerable human pathogen due to its capacity to cause numerous life-threatening diseases. As such S. aureus has an intricate metabolism that promotes proliferation in distinct host environments. However, little is known regarding the sulfur sources the pathogen scavenges from host tissues. Sulfur is an essential nutrient due to its extensive redox capacity and consequently, it is a critical component of many cofactors. Prior studies started to define sulfur sources S. aureus can use including the inorganic sulfur sources, sulfide and thiosulfate, and organosulfur sources, cysteine, cystine, and glutathione. While we understand some of the sulfur sources S. aureus can use, we do not know the genetic determinants facilitating assimilation. The present studies sought to explain how S. aureus imports and catabolizes organic sources of sulfur. First, we wanted to uncover the proteins allowing S. aureus to utilize cystine and cysteine as sulfur sources. The S. aureus homologues of characterized cystine transporters, TcyP and TcyABC, were experimentally validated as cystine and cysteine transporters. We expanded the sulfur sources S. aureus can utilize to include homocystine and N-acetyl cysteine and show that both TcyABC and TcyP support growth on N-acetyl cysteine while only TcyABC supports growth on homocystine. Finally, a tcyP mutant is impaired in murine heart and liver when competing with WT S. aureus suggesting import of TcyP substrates is important for heart and liver colonization. While a tcyP mutant is reduced in competition with WT in murine heart and liver colonization is not fully ablated signifying more sulfur sources must be catabolized. We next examined how S. aureus imports and catabolizes GSSG. To identify S. aureus GSSG utilization strategies, we used a chemically defined medium containing GSSG as the sulfur source and isolated mutants harboring transposon insertions within a putative ABC-transporter and -glutamyl transpeptidase that fail to proliferate. The mutants also do not grow in medium supplemented with GSH. Consistent with these findings, we named the locus the glutathione import system (gisABCD-ggt). Biochemical analysis of recombinant Ggt confirms in silico functional predictions by demonstrating that Ggt cleaves both GSH and GSSG. Though Gis mutants display wildtype virulence, we find that Gis-Ggt promotes competition with Staphylococcus epidermidis when GSH or GSSG is supplied as the sole sulfur source in vitro. S. aureus resides as a nasal commensal in 30% of the population and once inside the body can infect nearly every organ. Throughout the changing host environments, S. aureus must sense and acclimate to nutrient availability. We sought to define how sulfur starvation and growth on different sulfur sources changes the transcriptional profile of S. aureus. We described the transcriptional changes when WT S. aureus or a CymR, the sulfur transcriptional regulator, mutant were grown in sulfur replete and deplete conditions. We show sulfur starvation leads to significant expression changes including upregulation of iron acquisition encoding genes and oxidative stress encoding genes. Furthermore, we provide evidence showing upregulation of CymR dependent sulfur transporters when S. aureus is grown on GSH and thiosulfate both of which are conditions in which CymR repression should be occurring. Finally, this dissertation ends with areas of future exploration of sulfur source utilization in S. aureus. These avenues include examining nutrient sulfur available in distinct infection sites and expansion of the sulfur sources S. aureus can use. Overall, the work presented here substantially contributes to our understanding of what sulfur sources S. aureus imports and catabolizes and how different sulfur sources change the transcriptional states of the cell.
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