The staphylococcal virulon is controlled largely by the agr locus, a global accessory gene regulator that is autoinduced by a self-coded peptide (AIP) and is therefore a quorum sensor. The agr locus has diverged within and between species, giving rise to AIP variants that inhibit heterologous agr activation, an effect with therapeutic potential against Staphylococcus aureus: a single dose of an inhibitory AIP blocks the formation of an experimental murine abscess. As the AIP is unstable at physiological pH, owing to its essential thiolactone bond, its single-dose efficacy seems paradoxical, which has led us to analyze the in vivo kinetics of agr activation and the consequences of its blockage by a heterologous AIP. Initially, the infecting bacteria grow rapidly, achieving sufficient population density within the first 3 h to activate agr, and then enter a neutrophil-induced metabolic eclipse lasting for 2-3 d, followed by agr reactivation concomitantly with the development of the abscess. The inhibitory AIP prevents agr expression only during its short in vivo lifetime, suggesting that the agr-induced and therefore quorum-dependent synthesis of virulence factors shortly after infection is necessary for the subsequent development of the abscess lesion and bacterial survival. We confirm this finding by showing that a sterile agr ؉ supernatant causes a sterile abscess similar to the septic abscess caused by live bacteria. These results may provide a biological rationale for regulation of virulence factor expression by quorum sensing rather than by response to specific host signals.bioluminescent imaging ͉ in vivo gene expression ͉ bacterial pathogenesis A lthough there is a large body of information on the in vitro regulation of bacterial accessory genes involved in pathogenesis (referred to here as the virulon), the in vitro environment is highly artificial and applies only imperfectly, at best, to the real-life situation. To date, however, most in vivo approaches have generated single-point transcription profiles for infecting organisms but only a handful have addressed the expression pattern of virulence determinants through time, a critical feature of any infection (1 Agr is a complex locus that controls expression of a substantial part of the staphylococcal virulon (3, 4) consistent with its central role in pathogenesis (5-8). It consists of two divergent transcription units, driven by promoters P2 and P3 (9). The P2 operon contains four genes, agrB, agrD, agrC, and agrA, all of which are required for transcriptional activation of the agr system. AgrC is the receptor and AgrA the response regulator of a two-component signal transduction module that is autoinduced by a posttranslationally modified small peptide (AIP) (10, 11), processed by AgrB from the 46-residue agrD propeptide (12). The primary function of this four-gene unit is to activate the two major agr promoters, P2 and P3, significantly aided by a second regulatory protein, SarA (13, 14). The actual effector of agrdependent exoprotein gene regulation,...