Global regulator with both positive and negative effects that controls the expression of several virulence factors and the biofilm formation process in a cell density-dependent manner (By similarity). Required for transcription of primary transcripts RNAII and RNAIII generated by agr (virulence accessory gene regulator) locus. Negatively regulates the expression of spa (protein A) and aur (metalloprotease aureolysin). Homodimer. Belongs to the SarA family.