A perfect alliance: orchestrated interplay between quorum sensing and the type VI secretion system

Quorum sensing (QS) and the Type VI Secretion System (T6SS) constitute major regulatory and effector modules that, in numerous Gram-negative bacteria, couple bacterial communication with contact-dependent secretion. A growing body of evidence suggests that these systems are interconnected via multilayered regulatory networks, as opposed to a single universal circuit. Specifically, QS can govern T6SS expression through LuxR-family regulators, Qrr small RNAs, RpoN, cyclic di-GMP, and biofilm-associated spatial cues. Conversely, T6SS activity can indirectly modulate QS outputs by reshaping local community composition, restricting QS-defective mutants, or altering the proportions of signal-producing and signal-responding populations. The direction, magnitude, and biological outcomes of these interactions are highly contingent upon species, T6SS subtype, strain background, ecological niche, and experimental model. In this review, we synthesize the current body of evidence for direct and indirect QS-T6SS crosstalk, critically appraise unresolved controversies and knowledge gaps, and discuss how this regulatory interface might guide the development of anti-virulence strategies that target both bacterial communication and contact-dependent secretion.