The Increased Accumulation of Staphylococcus aureus Virulence Factors Is Maximized in a <i>purR</i> Mutant by the Increased Production of SarA and Decreased Production of Extracellular Proteases

Abstract

Mutation of <i>purR</i> was previously shown to enhance the virulence of <i>Staphylococcus aureus</i> in a murine sepsis model, and this cannot be fully explained by increased expression of genes within the purine biosynthesis pathway. Rather, the increased production of specific <i>S. aureus</i> virulence factors, including alpha toxin and the fibronectin-binding proteins, was shown to play an important role. Mutation of <i>purR</i> was also shown previously to result in increased abundance of SarA. Here, we demonstrate by transposon sequencing that mutation of <i>purR</i> in the USA300 strain LAC increases fitness in a biofilm while mutation of <i>sarA</i> has the opposite effect. Therefore, we assessed the impact of <i>sarA</i> on reported <i>purR</i>-associated phenotypes by characterizing isogenic <i>purR</i>, <i>sarA</i>, and <i>sarA/purR</i> mutants. The results confirmed that mutation of <i>purR</i> results in increased abundance of alpha toxin, protein A, the fibronectin-binding proteins, and SarA, decreased production of extracellular proteases, an increased capacity to form a biofilm, and increased virulence in an osteomyelitis model. Mutation of <i>sarA</i> had the opposite effects on all of these phenotypes and, other than bacterial burdens in the bone, all of the phenotypes of <i>sarA</i>/<i>purR</i> mutants were comparable to those of <i>sarA</i> mutants. Limiting the production of extracellular proteases reversed all of the phenotypes of <i>sarA</i> mutants and most of those of <i>sarA/purR</i> mutants. We conclude that a critical component defining the virulence of a <i>purR</i> mutant is the enhanced production of SarA, which limits protease production to an extent that promotes the accumulation of critical <i>S. aureus</i> virulence factors.