Date Published: May 4, 2016
Publisher: Public Library of Science
Author(s): Heidi A. Crosby, Patrick M. Schlievert, Joseph A. Merriman, Jessica M. King, Wilmara Salgado-Pabón, Alexander R. Horswill, Michael Otto.
Staphylococcus aureus is a human commensal and opportunistic pathogen that causes devastating infections in a wide range of locations within the body. One of the defining characteristics of S. aureus is its ability to form clumps in the presence of soluble fibrinogen, which likely has a protective benefit and facilitates adhesion to host tissue. We have previously shown that the ArlRS two-component regulatory system controls clumping, in part by repressing production of the large surface protein Ebh. In this work we show that ArlRS does not directly regulate Ebh, but instead ArlRS activates expression of the global regulator MgrA. Strains lacking mgrA fail to clump in the presence of fibrinogen, and clumping can be restored to an arlRS mutant by overexpressing either arlRS or mgrA, indicating that ArlRS and MgrA constitute a regulatory pathway. We used RNA-seq to show that MgrA represses ebh, as well as seven cell wall-associated proteins (SraP, Spa, FnbB, SasG, SasC, FmtB, and SdrD). EMSA analysis showed that MgrA directly represses expression of ebh and sraP. Clumping can be restored to an mgrA mutant by deleting the genes for Ebh, SraP and SasG, suggesting that increased expression of these proteins blocks clumping by steric hindrance. We show that mgrA mutants are less virulent in a rabbit model of endocarditis, and virulence can be partially restored by deleting the genes for the surface proteins ebh, sraP, and sasG. While mgrA mutants are unable to clump, they are known to have enhanced biofilm capacity. We demonstrate that this increase in biofilm formation is partially due to up-regulation of SasG, a surface protein known to promote intercellular interactions. These results confirm that ArlRS and MgrA constitute a regulatory cascade, and that they control expression of a number of genes important for virulence, including those for eight large surface proteins.
Staphylococcus aureus is a human commensal that asymptomatically colonizes the nares, throat, and skin of ~30% of the population [1,2]. It is also a pervasive opportunistic pathogen that is the most common infectious agent isolated from hospital inpatients in the US . S. aureus causes a range of diseases, from skin and soft tissue infections to life-threatening conditions like pneumonia, osteomyelitis, sepsis and infective endocarditis. Antibiotic resistance has been increasing among S. aureus isolates in the past few decades , limiting the available treatment options. For example, invasive infections caused by methicillin-resistant S. aureus (MRSA) have mortality rates approaching 20% , highlighting the need for innovative therapies.
The ArlRS TCS has been linked to virulence phenotypes multiple times [31–33], but we are only beginning to understand how this regulatory system functions. We demonstrate here that ArlR activates expression of the global regulator MgrA, and that mgrA mutants, like arlRS mutants, show a pronounced defect in clumping in the presence of fibrinogen. Numerous studies, including this one, have shown that MgrA is important for virulence [42,52–54,58], although the reason for this has been largely unclear. Our RNA-seq findings indicate MgrA affects the expression of >100 genes, including eight surface proteins that are likely to be important for adhesion and immune evasion within the host. These results support the idea that ArlRS and MgrA constitute a regulatory cascade that, in response to an unknown signal, profoundly changes expression of cell wall associated proteins, perhaps allowing S. aureus to adapt to a new niche within the host or progress to a different disease stage. A proposed model for how the ArlRS-MgrA cascade affects clumping and biofilm formation is shown in Fig 12.