Isolation of a Legionella pneumophila strain serologically distinguishable from all known serogroups

Comparative Genomics Reveal That Host-Innate Immune Responses Influence the Clinical Prevalence of Legionella pneumophila Serogroups

Legionella pneumophila is the primary etiologic agent of legionellosis, a potentially fatal respiratory illness. Amongst the sixteen described L. pneumophila serogroups, a majority of the clinical infections diagnosed using standard methods are serogroup 1 (Sg1). This high clinical prevalence of Sg1 is hypothesized to be linked to environmental specific advantages and/or to increased virulence of strains belonging to Sg1. The genetic determinants for this prevalence remain unknown primarily due to the limited genomic information available for non-Sg1 clinical strains. Through a systematic attempt to culture Legionella from patient respiratory samples, we have previously reported that 34% of all culture confirmed legionellosis cases in Ontario (n = 351) are caused by non-Sg1 Legionella. Phylogenetic analysis combining multiple-locus variable number tandem repeat analysis and sequence based typing profiles of all non-Sg1 identified that L. pneumophila clinical strains (n = 73) belonging to the two most prevalent molecular types were Sg6. We conducted whole genome sequencing of two strains representative of these sequence types and one distant neighbour. Comparative genomics of the three L. pneumophila Sg6 genomes reported here with published L. pneumophila serogroup 1 genomes identified genetic differences in the O-antigen biosynthetic cluster. Comparative optical mapping analysis between Sg6 and Sg1 further corroborated this finding. We confirmed an altered O-antigen profile of Sg6, and tested its possible effects on growth and replication in in vitro biological models and experimental murine infections. Our data indicates that while clinical Sg1 might not be better suited than Sg6 in colonizing environmental niches, increased bloodstream dissemination through resistance to the alternative pathway of complement mediated killing in the human host may explain its higher prevalence.

Investigation of the population structure of Legionella pneumophila by analysis of tandem repeat copy number and internal sequence variation

The population structure of the species Legionella pneumophila was investigated by multilocus variable number of tandem repeats (VNTR) analysis (MLVA) and sequencing of three VNTRs (Lpms01, Lpms04 and Lpms13) in selected strains. Of 150 isolates of diverse origins, 136 (86 %) were distributed into eight large MLVA clonal complexes (VACCs) and the rest were either unique or formed small clusters of up to two MLVA genotypes. In spite of the lower degree of genome-wide linkage disequilibrium of the MLVA loci compared with sequence-based typing, the clustering achieved by the two methods was highly congruent. The detailed analysis of VNTR Lpms04 alleles showed a very complex organization, with five different repeat unit lengths and a high level of internal variation. Within each MLVA-defined VACC, Lpms04 was endowed with a common recognizable pattern with some interesting exceptions. Evidence of recombination events was suggested by analysis of internal repeat variations at the two additional VNTR loci, Lpms01 and Lpms13. Sequence analysis of L. pneumophila VNTR locus Lpms04 alone provides a first-line assay for allocation of a new isolate within the L. pneumophila population structure and for epidemiological studies.