Legionella pneumophila is ubiquitous and sporadically infects humans causing legionnaires disease, a severe form of pneumonia. Globally, the number of reported cases of Legionnaire’s disease has risen four-fold from 2000-2014. In 2016, Sydney, Australia was the epicentre of an outbreak caused by Legionella pneumophila serogroup 1 (Lp1). The causal Lp1 clone was typed as sequence type (ST) 211, an ST only ever reported in Canada. This discovery raised questions about whether this clone was new or whether it had existed in the environment previously. However there is very little known about outbreak related strains in Australia with most studies concentrating on localised outbreaks or those occurring overseas. Genomic surveillance of Lp1 would contribute to our knowledge of its evolution and can increase our ability to assist public health control measures in the event of an outbreak.
Therefore whole genome sequencing (WGS) was employed to investigate historical isolates from both clinical and environmental samples with the aim of determining the major outbreak clones in Sydney and whether there are other factors involved in outbreak related strains through analysis of the pangenome.
WGS of 87 Lp1 isolates from 1994-2015 was performed and the genomes typed using the L. pneumophila Sequenced Based Typing Scheme. Significantly, ST211 was found in both clinical and environmental samples since 1994. A further common outbreak strain, ST37 was also found however it was not as dominant as ST211 yet interestingly, both of these STs had highly related pangenomes. This study highlights the added value of genomic surveillance of pathogens with outbreak potential such as Lp1. It has long been thought that ST1 was the dominant clone, however this study shows ST211 is predominate in NSW and highlights the genomic similarity of this clone from Australia to another major disease causing clone ST37 from the USA and Europe.