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ABSTRACT Quorum sensing (QS) is a widespread signaling mechanism in bacteria that coordinates collective behaviors according to population density. A foundational assumption in this field is that QS functions as a gene expression switch that synchronizes responses at the population level. While some studies indeed report homogeneous on/off transitions, others report heterogeneity at the cellular level, challenging the canonical view. To provide a formal context for these observations, we examined QS behavior at physiological steady state in the model bacteriumPseudomonas aeruginosa. We measured the expression of a centrallas-system component, the positively autoregulated signal synthase genelasI, at the population and single-cell level. In support of the canonical view and predictions from mathematical modeling, we show that thelassystem exhibits population-level bistability, with the entire unimodal population of cells switching synchronously between two stable states. We also show that bistable state switching exhibits hysteresis, indicative of memory within the system, with induced cells maintaining activation at considerably lower densities than previously uninduced cells. We confirm these behaviors in a subset of otherlas-QS controlled genes. Our study experimentally proves population-level bistability as a central, emergent property in a native QS system, with implications for physiology, pathogenesis, and synthetic biology.IMPORTANCEThis research investigates quorum sensing (QS), a common bacterial communication mechanism that controls processes like virulence, biofilm formation, and microbial warfare. Our study experimentally proves the long-standing notion that native QS can function as a true genetic switch that synchronizes all-or-none responses in a population of cells. We employ the well-understood LasI/LasR QS system of the opportunistic pathogenPseudomonas aeruginosaas a model. We show that the switch is bistable, with stable on and off states, and that it is hysteretic, with a type of memory that makes state-switching dependent on the initial condition. These properties impart robustness and stability to environmental changes akin to cellular developmental pathways; they have general implications for infection and its control, as well as genetic circuit design.