Abstract
Legionella species inhabit freshwater and soil ecosystems where they parasitize protozoa. L. pneumonphila (LP) serogroup-1 (Lp1) is the major cause of Legionnaires’ Disease (LD), a life-threatening pulmonary infection that can spread systemically. The increased global frequency of LD caused by Lp and non-Lp species underscores the need to expand our knowledge of evolutionary forces underlying disease pathogenesis. Whole genome analyses of 43 strains, including all known Lp serogroups 1–17 and 17 emergent LD-causing Legionella species (of which 33 were sequenced in this study) in addition to 10 publicly available genomes, resolved the strains into four phylogenetic clades along host virulence demarcations. Clade-specific genes were distinct for genetic exchange and signal-transduction, indicating adaptation to specific cellular and/or environmental niches. CRISPR spacer comparisons hinted at larger pools of accessory DNA sequences in Lp than predicted by the pan-genome analyses. While recombination within Lp was frequent and has been reported previously, population structure analysis identified surprisingly few DNA admixture events between species. In summary, diverse Legionella LD–causing species share a conserved core-genome, are genetically isolated from each other, and selectively acquire genes with potential for enhanced virulence.
Highlights
Legionella species inhabit freshwater and soil ecosystems where they parasitize protozoa
To improve our understanding of evolutionary forces acting on Legionella species, we performed comparative genomic analyses to elucidate population structure and estimate the effects of homologous recombination
The analysis set consisted of 10 published and 33 Legionella genomes sequenced in this study, including Legionella species occasionally reported as etiologic agents of human disease and Lp subtypes 2 through 17 (Table 1)
Summary
Legionella species inhabit freshwater and soil ecosystems where they parasitize protozoa. Whole genome analyses of 43 strains, including all known Lp serogroups 1–17 and 17 emergent LD-causing Legionella species (of which 33 were sequenced in this study) in addition to 10 publicly available genomes, resolved the strains into four phylogenetic clades along host virulence demarcations. L. pneumophila serogroup 1 (Lp1) is the leading cause of LD, accounting for up to 92% of clinically recognized legionellosis infections in the US and Europe[1]. Other species of Legionellae rarely cause disease except for L. longbeachae where high rates of infection and disease have been reported in Australia and New Zealand[8]. In 2015, there were 15 outbreaks in the US and Europe with a 10% case fatality rate in addition to the growing number of cases caused by non- Lp species. These collective findings drive home the critical need to better characterize legionellae to improve our understanding of their biology and epidemiology to advance the design of strategic interventions
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