Victor J. Torres

Staphylococcus aureus is an opportunistic pathogen that employs an array of different virulence factors to evade the immune system. The bi-component pore-forming leukocidins are of particular importance for their ability to target and kill phagocytes. The operon of two of these toxins, lukED and lukSF-PV , are repressed by two proteins, SarS and Rot. However, how these repressors work together to repress these toxins is not completely understood. Here, we determine that repression of leukocidins by SarS and Rot is not additive and that SarS and Rot are able to bind concurrently to the leukocidin promoters. In addition, in a tissue culture model using primary human neutrophils, the deletion of both repressors did not result in increased virulence compared to the single deletions of sarS and rot . Further experiments revealed that in an in vivo mouse infection model, virulence was similar in strains lacking one versus both repressors. Overall, these data show that while the repression of leukocidins by SarS and Rot is not additive, both proteins are critical for the repression of these toxins. IMPORTANCE The leukocidins play an important role in disarming the host immune system and promoting infection. While both SarS and Rot have been established as repressors of leukocidins, the importance of each repressor in infection is unclear. Here, we demonstrate that repression by SarS and Rot is not additive and show that in addition to upregulating expression of each other, they are also able to bind concurrently to the leukocidin promoters. These findings suggest that both repressors are necessary for maximal repression of lukED and lukSF-PV and illuminate another complex relationship among Staphylococcus aureus virulence regulators.