A quantitative map of beta-lactone induced virulence regulation

Beta-lactones have recently been introduced as the first selective ClpP inhibitors that attenuate virulence of both sensitive Staphylococcus aureus and multiresistant strains (MRSA). Although previous knockout studies showed that ClpP is essential for S. aureus alpha-toxin production a link between beta-lactone inhibition and molecular virulence mechanisms has been lacking so far. We here perform a chemical-proteomic approach to elucidate anti-virulence pathways. First, we demonstrate by gel-free activity-based protein profiling that ClpP is the predominant target of beta-lactones. Only a few off-targets were discovered, which, unlike ClpP, were not involved in the reduction of alpha-toxin expression. Second, in-depth mechanistic insight was provided by a full proteomic comparison between lactone treated and untreated S. aureus cells. Quantitative mass spectrometric analysis revealed a strong up-regulation of Rot and a corresponding down-regulation of alpha-toxin, providing the first direct connection between the lactone-dependent phenotype and a corresponding cellular mechanism. By building up a quantitative virulence regulation network, we visualize the impact of ClpP inhibition in a systems biology context. Interestingly, a lack of in vitro Rot degradation by either ClpXP or ClpCP calls for an indirect mode of action that may involve ClpX-mediated RNA signaling and feed-back circuits.