Staphylococcus aureus was recently shown to be internalized by and to induce apoptosis in a bovine mammary epithelial cell line, suggesting that these processes could be involved in staphylococcal pathogenesis or persistence. To examine the role of virulence factor regulators during internalization, mutantagr and sar strains of S. aureuswere analyzed for their abilities to enter and induce apoptosis in epithelial cells. Like a previously characterized bovine mastitis isolate, the standard laboratory strain, RN6390 (wild type), entered the epithelial cells and subsequently induced apoptosis. In contrast, the mutant strains RN6911 (agr), ALC136 (sar), and ALC135 (agr sar) were internalized by the cultured cells at levels reproducibly greater than that for RN6390 but failed to induce apoptosis. The internalization of S. aureus was affected by growth phase, suggesting a role foragr-regulated surface proteins in this process. Furthermore, the ability to induce apoptosis required metabolically active intracellular bacteria. These data indicate that the ability ofS. aureus to enter mammalian cells and induce apoptosis is dependent on factors regulated by Agr and Sar. Since transcriptional control by these global regulators is mediated by quorum-sensing and environmental factors, staphylococci may have the potential to induce several alternative effects on cells from an intracellular environment. A model for the function of the agr locus in the context of internalization, intracellular persistence, and dissemination is proposed.