Genomic analysis of Legionella pneumophila in the drinking water system of a large building over 25 years

<i>Legionella pneumophila</i>, the causative agent of Legionnaires' disease, is often found in the plumbing systems of buildings, from where it can be transmitted to humans via inhalation or aspiration of contaminated water drops. Annual routine water sampling from the potable water system of an occupational healthcare building in Basel over 25 years was performed in accordance with national guidelines. Overall, 309 water samples were collected at 38 time points over the period of 25 years. <i>L. pneumophila</i> was recovered from 120 water samples (38.8%) from 26 time points. No clinical infections were recorded during this period. Initial decontamination measures were successful for ~12 years, after which an increase in the total number of <i>Legionella</i> c.f.u. as well as of <i>L. pneumophila</i>-positive sites was noticed in 2007. Whole genome sequencing (WGS) analysis of <i>n</i>=123 isolates from <i>n</i>=113 samples showed all <i>L. pneumophila</i> to be sequence type 45 (sequence-based typing scheme). The isolates are closely related, with only 408 SNPs among all isolates after the bioinformatic removal of recombination events. Over the 25 years, a single lineage deriving from a recent common ancestor colonized the water system of this building. The phylogeny of isolate genomes can be interpreted as inferring good water circulation and possible recolonization from a common source after cleaning, with genome evolution and insertion/loss of large elements evident. Regular monitoring of waterlines in healthcare settings helps to identify concentrations of <i>Legionella</i> spp., and WGS is recommended for detailed investigation.