Regulation of Staphylococcus aureus α-toxin gene (hla) expression by agr, sarA, and sae in vitro and in experimental infective endocarditis

Background. The Staphylococcus aureus global regulators-agr, sarA, and sae-coordinately control a-toxin gene (hla) expression in vitro. However, their relative in vivo contributions to hla expression, particularly in endovascular infections, have not been defined. Methods. A plasmid-based h la-promoter: green fluorescent protein reporter system was constructed in 2 genetically related S. aureus strains: RN6390 (a natural sigma factor B [sigB]-deficient mutant), SH1000 (a sigB-repaired variant of RN6390 lineage), and their respective agr, sarA, agr/sarA, and sae mutants. These strain sets were used to quantify hla expression in vitro and in an experimental infective endocarditis (IE) model using flow cytometry. Results. In vitro, hla expression was positively modulated by all 3 regulons (sae > agr1sarA > agr and sarA) in both RNT6390 and SH1000 backgrounds. In the IE model, hla expression in cardiac vegetations was lower in all sin-le mutants than in the respective parental strains (P < .05 for sae mutant) but was maintained at near-parental levels in the agr/sai-A double mutant in both backgrounds. A similar finding was also observed in kidneys and spleens. Conclusions. These results indicate that, although several distinct regulatory circuits can affect hla expression in vitro and in vivo, sae appears to play a crucial role in this context.