Quorum-sensing <i>agr</i> system of <i>Staphylococcus aureus</i> primes gene expression for protection from lethal oxidative stress.

Abstract

The <i>agr</i> quorum-sensing system links <i>Staphylococcus aureus</i> metabolism to virulence, in part by increasing bacterial survival during exposure to lethal concentrations of H₂O₂, a crucial host defense against <i>S. aureus</i>. We now report that protection by <i>agr</i> surprisingly extends beyond post-exponential growth to the exit from stationary phase when the <i>agr</i> system is no longer turned on. Thus, <i>agr</i> can be considered a constitutive protective factor. Deletion of <i>agr</i> resulted in decreased ATP levels and growth, despite increased rates of respiration or fermentation at appropriate oxygen tensions, suggesting that Δ<i>agr</i> cells undergo a shift towards a hyperactive metabolic state in response to diminished metabolic efficiency. As expected from increased respiratory gene expression, reactive oxygen species (ROS) accumulated more in the <i>agr</i> mutant than in wild-type cells, thereby explaining elevated susceptibility of Δ<i>agr</i> strains to lethal H₂O₂ doses. Increased survival of wild-type <i>agr</i> cells during H₂O₂ exposure required <i>sodA</i>, which detoxifies superoxide. Additionally, pretreatment of <i>S. aureus</i> with respiration-reducing menadione protected Δ<i>agr</i> cells from killing by H₂O₂. Thus, genetic deletion and pharmacologic experiments indicate that <i>agr</i> helps control endogenous ROS, thereby providing resilience against exogenous ROS. The long-lived 'memory' of <i>agr</i>-mediated protection, which is uncoupled from <i>agr</i> activation kinetics, increased hematogenous dissemination to certain tissues during sepsis in ROS-producing, wild-type mice but not ROS-deficient (<i>Cybb</i>^-/-) mice. These results demonstrate the importance of protection that anticipates impending ROS-mediated immune attack. The ubiquity of quorum sensing suggests that it protects many bacterial species from oxidative damage.