Lay summary
Background: Staphylococcus aureus remains one of the most challengingmicroorganisms causing a wide spectrum of diseases ranging from benign tosevere life-threatening infections in humans. Staphylococcal infectionsare difficult to handle, especially in the presence of implantedprostheses or catheters, and further compromised by multiple resistance toseveral antibiotics. The mechanisms allowing S. aureus to be a mostsuccessful pathogen in hospital and community environments include: (i) alarge repertoire of virulence factors that are either cell-associated(promoting attachment to host components) or released (host-damagingexotoxins or proteases); (ii) the controlled and coordinated expression ofmost virulence factors by an intricate and very elaborated network ofglobal regulators; (iii) the outstanding ability to survive suboptimal oreven extreme growth conditions inside or outside their host by a finelytuned, though poorly explored, network of stress response pathways.

Working hypothesis: To explain how very few methicillin-resistant S.aureus (MRSA) lineages have become prevalent in the hospital environmentand more recently in the community, we consider that these isolates arosefrom a multistep selection process. We speculate that these strainsacquired an optimal combination of virulence factors, multiple antibioticresistance determinants, and efficient stress response pathways. Theseselected properties were either developed endogenously by spontaneousmutations under stress pressure or acquired by lateral transfer ofdiscrete genetic elements such as pathogenicity or/and antibioticresistance islands.

Specific aims: This proposal aims to identify or further characterize bycombined molecular and functional approaches major stress responsepathways that may contribute to regulated expression of virulencedeterminants, optimized survival, and enhanced S. aureus dissemination.
Improved knowledge of stress response and DNA repair mechanisms selectingfor clinical isolates highly adapted to the hospital environment shouldhelp elaborating improved hospital infection control strategies andrevealing novel targets for antimicrobial chemotherapy.

Experimental Design and Methods: Identification or furthercharacterization of DNA repair and stress response components and pathwayswill be performed by gene expression profiles of stress-exposed bacteriamonitored by DNA microarrays. To facilitate characterization of stressresponse pathways, we will initially use a simplified model ofstress-exposed bacteria, consisting in quinolone-resistant mutants thatwere found to simultaneously exhibit increased attachment to fibronectinand improved survival at high temperature compared to their isogenicquinolone-susceptible parents. Selected components of each identifiedresponse pathway will be further studied by quantitative real-time RT-PCRand targeted mutagenesis. In a 2nd step, some highly prevalent hospitalisolates will be analyzed for expression of stress response pathways incorrelation with virulence and antimicrobial resistance determinants.