Inhibition of Staphylococcal Cell-To-Cell Communication and Virulence by the Innate Immunity Mediator Nitric Oxide

Abstract

Nitric oxide (NO) is a major mediator of host innate immunity with antimicrobial activity against a broad range of pathogens. Specific targeting of protein metal centers, thiols, and other radicals can disrupt microbial metabolism and limit pathogen growth during infection. The opportunistic pathogen Staphylococcus aureus is relatively resistant to NO-mediated growth inhibition, yet NO remains important to control infection. A possible mechanism by which host NO is protective, beyond growth inhibition, may be through the direct targeting of systems that regulate the production of virulence factors, such as toxins. In Staphylococcus aureus, cell-to-cell communication known as quorum sensing regulates virulence and determines whether interactions with a mammalian host are commensal or pathogenic. Despite the importance of quorum sensing to infection, little is known about how host immunity affects inter-bacterial communication. In this thesis, I show that NO, a bacteriostatic effector of innate immunity, suppresses virulence by targeting the staphylococcal Agr quorum sensing system. Inhibition of Agr results from the direct modification of cysteine residues C55, C123, and C199 of the AgrA transcription factor. Cysteine modification decreases AgrA promoter occupancy and transcription of the agr operon and quorum sensing-activated toxin genes. In a murine model of staphylococcal pneumonia, mice lacking inducible NO synthase developed more severe disease, elicited higher pro-inflammatory cytokine responses, and displayed different histopathology compared to wild-type mice. These findings points toward a novel anti-virulence role for NO.