Gene flow in environmental Legionella pneumophila leads to genetic and pathogenic heterogeneity within a Legionnaires' disease outbreak.

Paul R Mcadam,Giles F Edwards,Joanne M Stevens,Michael Gillies,Diane S J Lindsay,Melissa J Ward,Mary F Hanson,Charles W Vander Broek,J Fitzgerald,Michael Watson

doi:10.1186/preaccept-1675723368141690

Abstract

Legionnaires' disease is a severe form of pneumonia caused by the environmental bacterium Legionella pneumophila. Outbreaks commonly affect people with known risk factors, but the genetic and pathogenic complexity of L. pneumophila within an outbreak is not well understood. Here, we investigate the etiology of the major Legionnaires' disease outbreak that occurred in Edinburgh, UK, in 2012, by examining the evolutionary history, genome content, and virulence of L. pneumophila clinical isolates. Our high resolution genomic approach reveals that the outbreak was caused by multiple genetic subtypes of L. pneumophila, the majority of which had diversified from a single progenitor through mutation, recombination, and horizontal gene transfer within an environmental reservoir prior to release. In addition, we discover that some patients were infected with multiple L. pneumophila subtypes, a finding which can affect the certainty of source attribution. Importantly, variation in the complement of type IV secretion systems encoded by different genetic subtypes correlates with virulence in a Galleria mellonella model of infection, revealing variation in pathogenic potential among the outbreak source population of L. pneumophila. Taken together, our study indicates previously cryptic levels of pathogen heterogeneity within a Legionnaires' disease outbreak, a discovery that impacts on source attribution for future outbreak investigations. Furthermore, our data suggest that in addition to host immune status, pathogen diversity may be an important influence on the clinical outcome of individual outbreak infections.

Highlights

Legionnaires’ disease is a severe form of pneumonia caused by the environmental bacterium Legionella pneumophila
An additional three contemporaneous clinical isolates that were epidemiologically unrelated to the Edinburgh outbreak, four environmental isolates of ST191 obtained previously in the UK, and a single Sg 1, ST591 isolate were sequenced to provide a phylogenetic context to the outbreak isolates (Table 1)
Variation in type IV secretion system (T4SS) gene content among outbreak isolates correlates with virulence in a G. mellonella infection model The T4SSs play a central role in the capacity of L. pneumophila to infect free-living amoeba and survive within vacuoles in human alveolar macrophages

Summary

Introduction

Legionnaires’ disease is a severe form of pneumonia caused by the environmental bacterium Legionella pneumophila. The bacterium can cause human infections (legionellosis) and is a causative agent of the severe community-acquired pneumonia known as Legionnaires’ disease [2]. The risk of human exposure to aerosols containing L. pneumophila increases with elevated ambient. The ability of L. pneumophila to establish infection within human host cells is mediated through type IV secretion systems (T4SSs), complexes of proteins homologous to conjugation systems that mediate the transfer of nucleoprotein complexes and proteins between cells [8,9]. T4ASS, encoded by the L. pneumophila Lvh locus, has a role in host-cell entry and intracellular replication [13], and contributes to establishment of infection at lower temperatures [14], while the Dot/Icm system encoded by a T4BSS is essential for intracellular replication [15,16,17]. The functions of many of the effector proteins are unclear but a large number demonstrate homology with eukaryotic proteins, which may allow L. pneumophila to modulate the host response through structural mimicry of host components [18,19,20,21]

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Conclusion