Quorum sensing beyond the virulence switch: communication, biofilm plasticity, and anti-virulence strategies in Staphylococcus aureus

The accessory gene regulator (agr) quorum-sensing system coordinates important aspects of Staphylococcus aureus virulence, including the expression of secreted toxins and enzymes and the transition between surface-associated and dispersal-associated behaviours. Yet, agr is not a universal binary switch: its output depends on agr type, strain background, environmental conditions, regulatory cross-talk, biofilm stage, host context, and the distribution of signalling states among individual cells. This review synthesized evidence on the molecular logic of agr, its context-dependent relationship with biofilm formation and dispersal, spatial and single-cell heterogeneity, host-pathogen interactions, and emerging anti-virulence approaches. Experimental studies demonstrate that agr signalling can promote biofilm dispersal, that disrupting biofilm organization can alter agr-regulated virulence expression, and that single-cell activation can be heterogeneous. Other findings, including direct host-signal sensing by regulatory systems in non-staphylococcal pathogens and the effects of quorum quenching in other species, are relevant comparative examples but do not establish equivalent mechanisms in S. aureus. The review therefore distinguishes direct S. aureus evidence from cross-species analogies and proposed therapeutic hypotheses. We use "communication-state engineering" as a conceptual framework for evaluating whether an intervention changes the distribution of bacterial phenotypes in a clinically favourable direction, rather than judging efficacy solely by reduced agr activity. Candidate approaches include autoinducing-peptide analogues, inhibitors of AgrA or associated regulatory nodes, natural products, microbial signalling interference, quorum-quenching enzymes, ClpP-directed anti-virulence agents, and combination treatments. Important limitations remain: much evidence is derived from in vitro systems or animal models, the effects of agr inhibition on biofilm persistence are context-dependent, and clinical benefit has not been established for agr-directed therapy. Progress requires temporally resolved biofilm assays, single-cell and spatial measurements, mechanistic controls for growth inhibition, and testing in relevant infection models.