The Staphylococcus aureus Global Regulator MgrA Modulates Clumping and Virulence by Controlling Surface Protein Expression

Abstract

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 Aggregation and Coagulation Mechanisms, and Their Function in Host–Pathogen Interactions

“…The ArlRS system has been previously linked to biofilm formation and virulence, although the mechanism of action was largely unknown (Benton et al, 2004; Liang et al, 2005; Toledo-Arana et al, 2005). It is becoming clear that ArlRS activates expression of the global regulator MgrA, which in turn controls expression of more than 100 genes (Crosby et al, 2016; Luong et al, 2006; Luong and Lee, 2006). Among the genes under the control of this regulatory cascade are those for eight surface proteins, several of which appear to be responsible for blocking clumping.…”

“…Among the genes under the control of this regulatory cascade are those for eight surface proteins, several of which appear to be responsible for blocking clumping. Specifically, MgrA represses expression of the large surface proteins Ebh, SraP, and SasG, and increased production of any one of these proteins can interfere with clumping (Crosby et al, 2016). We hypothesize that these proteins act is a similar fashion as Pls, and that when they are de-repressed they block interactions with fibrinogen through steric hindrance.…”

“…To further underscore that clumping and biofilm formation are distinct processes, strains lacking arlRS or mgrA are defective in clumping but actually produce increased amounts of biofilm (Crosby et al, 2016; Toledo-Arana et al, 2005; Trotonda et al, 2008; Walker et al, 2013). Upregulation of large surface proteins in these mutants appears to interfere with the ability of planktonic cells to approach each other and interact via fibrinogen bridges.…”

“…Yet increased production of one of these surface proteins, SasG, also seems to facilitate biofilm formation among surface-attached cells (Fig. 4B) (Crosby et al, 2016). SasG is known to promote biofilm formation (Geoghegan et al, 2010a), but it has not previously been appreciated that expression of proteins like SasG can simultaneously block clumping and promote biofilm formation, depending on the growth conditions and human matrix proteins present.…”

“…Strains that lack arlRS or mgrA , and thus are unable to clump, consistently show reduced virulence in models of endocarditis, sepsis, arthritis and skin abscesses (Benton et al, 2004; Chen et al, 2006; Crosby et al, 2016; Gupta et al, 2013; Liu et al, 2014; Walker et al, 2013). Also, mutations in genes of individual proteins responsible for clumping and/or coagulation, such as clfA , coa and vWbp , lead to decreased virulence.…”