Genomic analysis of the liverpool epidemic strain of pseudomonas aeruginosa infecting persons with cystic fibrosis reveals likely Canadian origins

INTRODUCTION: The Liverpool Epidemic Strain (LES) of Pseudomonas aeruginosa is one of several known strains to be transmissible between persons with cystic fibrosis (CF) (pwCF) and the only known strain to have infected large proportions of CF populations on two continents. Despite its prevalence, efforts to understand its spread have proven elusive. METHODS: We leveraged a prospective collection of P. aeruginosa isolates from pwCF attending the Southern Alberta Adult CF clinic from 1986 to 2020 to identify all individuals with LES infection. LES isolates collected every 1-2 years from each pwCF were sequenced and compared with 171 published LES genomes by phylogenetic analysis. RESULTS: Of 395 pwCF screened, ten pwCF infected with the LES were identified, from whom 46 LES isolates were sequenced. The earliest LES isolate was recovered in 1986, ∼2 years earlier than the previously oldest published LES isolate recovered in the UK. Phylogenetic analysis identified a diverse set of isolates at the root of the LES phylogeny that formed four clades, one of which gave rise to a "classic LES" clade. Canadian isolates formed a paraphyletic group that included the root of this clade and out of which the UK LES clade emerged. We estimated the date of the most recent common ancestor (MRCA) of the UK LES clade as 1977. CONCLUSIONS: Our study provides genomic evidence in support of a silent epidemic of LES infection occurring in the late 1970s among pwCF first originating in Canada and being spread to the UK, where transmission markedly accelerated.