Quorum sensing governs a transmissive Legionella subpopulation at the pathogen vacuole periphery.

The Gram-negative bacterium Legionella pneumophila is the causative agent of Legionnaires' disease and replicates in amoebae and macrophages within a distinct compartment, the Legionella-containing vacuole (LCV). The facultative intracellular pathogen switches between a replicative, non-virulent and a non-replicating, virulent/transmissive phase. Here, we show on a single-cell level that at late stages of infection, individual motile (PflaA -GFP-positive) and virulent (PralF - and PsidC -GFP-positive) L. pneumophila emerge in the cluster of non-growing bacteria within an LCV. Comparative proteomics of PflaA -GFP-positive and PflaA -GFP-negative L. pneumophila subpopulations reveals distinct proteomes with flagellar proteins or cell division proteins being preferentially produced by the former or the latter, respectively. Toward the end of an infection cycle (˜ 48 h), the PflaA -GFP-positive L. pneumophila subpopulation emerges at the cluster periphery, predominantly escapes the LCV, and spreads from the bursting host cell. These processes are mediated by the Legionella quorum sensing (Lqs) system. Thus, quorum sensing regulates the emergence of a subpopulation of transmissive L. pneumophila at the LCV periphery, and phenotypic heterogeneity underlies the intravacuolar bi-phasic life cycle of L. pneumophila.