TheLegionellaautoinducer LAI-1 is delivered by outer membrane vesicles to promote inter-bacterial and inter-kingdom signaling

Abstract

SummaryLegionella pneumophilais an environmental bacterium, which replicates in amoeba but also in macrophages, and causes a life-threatening pneumonia called Legionnaires’ disease. The opportunistic pathogen employs the α-hydroxyketone compound LAI-1 (Legionellaautoinducer-1) for intra-species and inter-kingdom signaling. LAI-1 is produced by the autoinducer synthase LqsA, but it is not known, how LAI-1 is released by the pathogen. Here, we use aV. choleraeluminescence reporter strain and liquid chromatography-tandem mass spectrometry (LC-MS/MS) to detect bacteria-produced and synthetic LAI-1. Ectopic production of LqsA inE. coligenerated LAI-1, which partitions to outer membrane vesicles (OMVs), and slightly reduces OMV size. TheseE. coliOMVs trigger luminescence of theV. choleraereporter strain and inhibit the migration ofDictyostelium discoideumamoeba. Overexpression oflqsAinL. pneumophilaunder the control of strong stationary phase promoters (PflaAor P6SRNA), but not under control of its endogenous promoter (PlqsA), produces LAI-1, which is detected in purified OMVs. TheseL. pneumophilaOMVs trigger luminescence of theVibrioreporter strain and inhibitD. discoideummigration.L. pneumophilaOMVs are smaller upon overexpression oflqsAor upon addition of LAI-1 to growing bacteria, and therefore, LqsA affects OMV production. The overexpression oflqsAbut not a catalytically inactive mutant promotes intracellular replication ofL. pneumophilain macrophages, indicating that intracellularly produced LA1-1 modulates the interaction in favour of the pathogen. Taken together, we provide evidence thatL. pneumophilaLAI-1 is secreted through OMVs and promotes inter-bacterial communication as well as interactions with eukaryotic host cells.Originality - Significance StatementInter-kingdom signaling involving low molecular weight bacterial compounds that are detected by eukaryotic cells represents an important, yet incompletely understood aspect of pathogen-host interactions. In many cases, the small signaling molecules are produced in only little amounts, their secretion mechanism is not known, and their effects on eukaryotic host cells are barely studied. Here, we reveal that the α-hydroxyketone compound LAI-1 ofL. pneumophilais released from the bacteria by outer membrane vesicles, which promote inter-bacterial communication as well as inter-kingdom signaling.