Synergistic Contribution of theLegionella pneumophila lqsGenes to Pathogen-Host Interactions

Abstract

The causative agent of Legionnaires' disease, Legionella pneumophila, is a natural parasite of environmental protozoa and employs a biphasic life style to switch between a replicative and a transmissive (virulent) phase. L. pneumophila harbors the lqs (Legionella quorum sensing) cluster, which includes genes encoding the autoinducer synthase LqsA, the sensor kinase LqsS, the response regulator LqsR, and a homologue of HdeD, which is involved in acid resistance in Escherichia coli. LqsR promotes host-cell interactions as an element of the stationary-phase virulence regulatory network. Here, we characterize L. pneumophila mutant strains lacking all four genes of the lqs cluster or only the hdeD gene. While an hdeD mutant strain did not have overt physiological or virulence phenotypes, an lqs mutant showed an aberrant morphology in stationary growth phase and was defective for intracellular growth, efficient phagocytosis, and cytotoxicity against host cells. Cytotoxicity was restored upon reintroduction of the lqs genes into the chromosome of an lqs mutant strain. The deletion of the lqs cluster caused more-severe phenotypes than deletion of only lqsR, suggesting a synergistic effect of the other lqs genes. A transcriptome analysis indicated that in the stationary phase more than 380 genes were differentially regulated in the lqs mutant and wild-type L. pneumophila. Genes involved in protein production, metabolism, and bioenergetics were upregulated in the lqs mutant, whereas genes encoding virulence factors, such as effectors secreted by the Icm/Dot type IV secretion system, were downregulated. A proteome analysis revealed that a set of Icm/Dot substrates is not produced in the absence of the lqs gene cluster, which confirms the findings from DNA microarray assays and mirrors the virulence phenotype of the lqs mutant strain.In the environment, Legionella pneumophila colonizes a wide range of aquatic habitats, including biofilms, but most prominently, the gram-negative bacteria survive as intracellular parasites of free-living amoebae (20). The eukaryotic phagocytes provide nutrients and protection against adverse conditions and may serve as transmission vectors (26, 32). The interactions between L. pneumophila and protozoa likely selected a pool of bacterial virulence traits that support bacterial survival and replication within mammalian phagocytes (8, 44). Indeed, upon inhalation of aerosols from contaminated water sources, L. pneumophila replicates within and kills alveolar macrophages (50), thereby causing inflammation and potentially evoking a life-threatening pneumonia termed Legionnaires' disease.L. pneumophila establishes its intracellular niche in host cells by forming Legionella-containing vacuoles (LCVs), wherein the bacteria are not degraded but rather replicate. To this end, LCVs avoid the endosomal pathway by inhibiting phagosomelysosome fusion and instead recruit early secretory vesicles and the endoplasmic reticulum (66). A key bacterial factor involved in efficient uptake and formation of LCVs is ...

Identification of Legionella pneumophila Effectors Regulated by the LetAS-RsmYZ-CsrA Regulatory Cascade, Many of Which Modulate Vesicular Trafficking

“…2A and B, respectively). It should be noted that FleR and LqsR were previously suggested to be involved in flagellum gene expression (57,58) and that LqsR was also suggested to be involved in the regulation of effector-encoding genes (58). These results show that, in addition to directly contributing to the regulation of effector-encoding genes, the LetA-RsmYZ-CsrA cascade also controls the expression of regulators that by themselves might regulate the expression of other effector-encoding genes.…”

Molecular Pathogenesis of Infections Caused byLegionella pneumophila

“…The response regulator LqsR contributes to virulence processes such as phagocytosis and the formation of the LCV by relaying signals from the adjacent sensor kinase LqsS (334,335). Neighboring this system is an autoinducer synthase, LqsA, which was recently shown to synthesize 3-hydroxypentadecan-4-one, the small Legionella autoinducer 1 (LAI1) (313).…”

Characterization of the tail-specific protease (Tsp) from Legionella

“…Bruggemann and co-workers examined the transcriptomic changes for the transmissive phase relative to the replicative phase of L. pneumophila Colby strain and found that tsp was up-regulated 54-fold (Bruggemann et al, 2006). Secondly, the Lqs (Legionella quorum sensing) gene is a key transcriptional activator for the transmissive phase and lqs mutants were found to have lower levels of tsp mRNA and protein (Tiaden et al, 2008). Tsp (tail-specific protease) was first identified from E. coli as a periplasmic serine protease which selectively degrades proteins with specific hydrophobic C-terminal sequences (Silber et al, 1992;Keiler and Sauer, 1995), impacting on cell division and morphology during stress (Hara et al, 1991), and has also been implicated in potentiating long-chain fatty acid transport in E. coli (Azizan and Black, 1994).…”