Accessory gene regulator system (agr) (agr system)

The Accessory gene regulator (agr) system is the primary quorum-sensing mechanism in Staphylococcus aureus, serving as a master regulator of virulence factor expression (Jenul & Horswill, 2019, PubMed: 30622368). This two-component system consists of four genes (agrA, agrB, agrC, and agrD) that produce and sense an autoinducing peptide (AIP). As the bacterial population density increases, the extracellular concentration of AIP rises; upon reaching a threshold, AIP binds to the transmembrane histidine kinase AgrC, which then activates the response regulator AgrA (Wang & Muir, 2016, PubMed: 27157113). Activated AgrA triggers the expression of RNAIII, the system's primary effector molecule, leading to the upregulation of secreted toxins, hemolysins, and proteases while downregulating surface adhesion proteins (Butrico & Cassat, 2020, PubMed: 32367500). In clinical disease, the agr system is critical for the transition from a commensal or colonization state to an invasive infection, contributing to tissue damage and immune evasion (Gordon et al., 2013, PubMed: 23616521). Because this system controls virulence without being essential for bacterial growth, it is a major target for anti-virulence therapy, which aims to 'disarm' the pathogen rather than kill it, potentially reducing the pressure for antibiotic resistance. Experimental drugs like Savirin and various AIP analogs function by inhibiting these signaling components, thereby attenuating toxin production and improving host immune clearance (Sully et al., 2014, PubMed: 25330349).

Other names

Staphylococcus aureus quorum sensing systemagr locusAIP-AgrC-AgrA signaling pathwayAccessory gene regulator protein systemAgrABCD

Inhibition of the Accessory gene regulator (agr) system typically occurs through several mechanisms: competitive antagonism of the AgrC receptor by autoinducing peptide (AIP) analogs or small molecules, which prevents the phosphorylation of the response regulator AgrA (Sully et al., 2014, PubMed: 25330349); or the direct inhibition of AgrA DNA-binding activity, which prevents the transcription of the RNAIII effector molecule and other virulence genes (Khodaverdian et al., 2015, PubMed: 26150534). Some agents may also disrupt the processing of AIP by the AgrB membrane protein (Jenul & Horswill, 2019, PubMed: 30622368).