SRA

Legionella pneumophila was first recognized as a cause of severe and potentially fatal pneumonia during a large-scale outbreak of Legionnaires' disease (LD) at a Pennsylvania veterans' convention in 1976. Here we describe archival research and the sequencing and comparative genomic analysis of all Philadelphia LD strains recovered from that investigation, along with L. pneumophila isolates sharing the Philadelphia sequence type (ST36). Analyses revealed that the 1976 outbreak was due to multiple serogroup 1 strains within the same genetic lineage, differentiated by a mobilizable episome and two large, horizontally-transferred genomic loci, among other polymorphisms. We also found that epidemiological context was critical to interpreting phylogenetic relationships among several ST36 strains that displayed surprising genetic similarity. These results, along with the identification of the “Broad Street pneumonia” isolate, provide new historical and genetic insights into the 1976 epidemic and may help inform Legionella outbreak investigations where whole-genome sequencing is employed.