Mastitis resulting from mammary gland infection is a common and painful disease associated with lactation. In addition to the impact on human and animal health, mastitis causes substantial economic losses in the dairy industry. Staphylococcus aureus is a frequent cause of mastitis worldwide. Despite significant progress in understanding S. aureus pathogenesis in general, much remains to be learned regarding virulence factors relevant in the context of mastitis. In mammary gland infections, it is not fully understood which metal acquisition systems are required for S. aureus survival. To help understand molecular mechanisms by which S. aureus might acquire essential metals, such as iron, within lactating mammary glands, S. aureus mutants were tested for growth defects in vitro. A low-iron media (TMM) was created and supplemented with differing iron sources relevant to mastitis infection such as host iron-binding proteins lactoferrin and transferrin. Mutants were grown in the various iron sources to determine which genes were involved with iron acquisition for each specific media tested. Results show that a double knock-out (∆htsA/sirA::ba) involved with two iron siderophore receptors and the ATPase, fhuC::ba, which powers those receptors are essential for growth in media supplemented with human lactoferrin, while mutants involved with the iron siderophore Staphyloferrin B (sbnE::ba) and its specific receptor (sirA::ba) proved important for growth in bovine lactoferrin. Additionally, S. aureus mutants were grown in bovine and human milk. Significant growth defects in human milk were found for mutants involved with zinc (znuBC::ba) and manganese (psaA::ba) acquisition. Iron limitations leading to growth defects were also found in ∆htsA/sirA::ba and fhuC::ba grown in human milk. Growth defects in bovine milk were seen for psaA::ba but not zinc genes. Growth of the fhuC mutant was shown to be significant, but not the double knock-out, indicating that iron acquisition in bovine milk does not involve the SirABC or HtsABC siderophore receptors. A mutant involved in purine synthesis, purH::ba, was also shown to have a significant growth defect in bovine milk. The importance of S. aureus metal acquisition has been well established, but there is a significant need to research these multifaceted processes further. Increased understanding of how metal acquisition facilitates bacterial survival in the lactating mammary gland can provide therapeutic targets for more effective mastitis prevention and treatment.

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