Genome analysis of Legionella pneumophila ST23, responsible for sporadic and epidemic cases in Italy, 1995-2018, reveals highly conservative strains

Legionella pneumophila serogroup 1 (Lp1) sequence type (ST) 23 is one of the most common worldwide detected and in Italy caused all outbreaks so far investigated. ST23 genomes, both causing epidemic and sporadic cases between 1995 and 2018, were analysed at genome level in order to determine possible similarities or differences.A core genome multi locus sequence typing (cgMLST), based on a previously described set of 1521 core genes, and single nucleotide polymorphisms (SNPs) approaches were applied to the Lp1 ST23 strain collection. DNAs were automatically extracted and libraries prepared using Nextera library kit and sequencing performed in MiSeq instrument. ST23 isolated in Italy, Denmark and Scottish were sequenced and their cgMLST determined.Both cgMLST and SNPs analyses showed very few loci and single nucleotide variations occurring in ST23 genomes. All the ST23 genomes causing the outbreaks occurred in Italy, as well as all those from sporadic cases, were very strictly phylogenetically related, independently on year or town of isolation. In addition, most of the variations were located in adjacent genomic regions and they were demonstrated to be mainly due to recombination events. The Italian ST23 were found to be highly conserved also compared with those from other countries.Lp1 ST23 isolated in Italy have conserved their monophyletic origin. As ST23 are quite widespread in Italy, a thorough epidemiological investigation is compelled to find the correct source of infection when the Lp1 ST23 is identified in both LD sporadic cases and outbreaks.