Identification of Legionella pneumophila genes important for infection of amoebas by signature-tagged mutagenesis

Legionella pneumophila is a facultative intracellular gram-negative rod that causes pneumonia in humans. Free-living amoebas are thought to serve as a reservoir for Legionella infections. Signature-tagged mutagenesis was employed to identify Legionella pneumophila genes necessary for survival in the amoeba Acanthamoeba castellanii. Six mutant strains were defective in assays of invasion and intracellular growth. Four mutants also exhibited invasion and replication defects in Hartmannella vermiformis, an amoeba linked to hospital outbreaks of Legionella pneumonia, The six mutants also were tested in macrophages derived from peripheral blood mononuclear cells. Two mutants had intracellular replication defects, and two different strains entered cells less efficiently. Two transposon insertions were in known L. pneumophila genes, lspK and aroB. The other four were in novel genes. One gene has similarity to a cytochrome c-type biogenesis protein of Pseudomonas fluorescens. Another has similarity to a transcriptional activator regulating flagellar biosynthesis in Vibrio cholera. The third is similar to traA of Rhizobium sp. strain NGR234, which is involved in conjugal transfer of DNA. The fourth has no homology. By using survival in amoeba as a selection, we have isolated mutant strains with a range of phenotypes; and we have potentially identified new L. pneumophila virulence genes.