<i>De Novo</i> Purine Biosynthesis Is Required for Intracellular Growth of Staphylococcus aureus and for the Hypervirulence Phenotype of a <i>purR</i> Mutant

Abstract

<i>Staphylococcus aureus</i> is a noted human and animal pathogen. Despite decades of research on this important bacterium, there are still many unanswered questions regarding the pathogenic mechanisms it uses to infect the mammalian host. This can be attributed to it possessing a plethora of virulence factors and complex virulence factor and metabolic regulation. PurR, the purine biosynthesis regulator, was recently also shown to regulate virulence factors in <i>S. aureus</i>, and mutations in <i>purR</i> result in derepression of fibronectin binding proteins (FnBPs) and extracellular toxins, required for a so-called hypervirulent phenotype. Here, we show that hypervirulent strains containing <i>purR</i> mutations can be attenuated with the addition of purine biosynthesis mutations, implicating the necessity for <i>de novo</i> purine biosynthesis in this phenotype and indicating that <i>S. aureus</i> in the mammalian host experiences purine limitation. Using cell culture, we showed that while <i>purR</i> mutants are not altered in epithelial cell binding, compared to that of wild-type (WT) <i>S. aureus</i>, <i>purR</i> mutants have enhanced invasion of these nonprofessional phagocytes, consistent with the requirement of FnBPs for invasion of these cells. This correlates with <i>purR</i> mutants having increased transcription of <i>fnb</i> genes, resulting in higher levels of surface-exposed FnBPs to promote invasion. These data provide important contributions to our understanding of how the pathogenesis of <i>S. aureus</i> is affected by sensing of purine levels during infection of the mammalian host.