Co-existence of multiple distinct lineages in serotype O4:K12.

is an important cause of foodborne gastroenteritis globally. Thermostable direct haemolysin (TDH) and the TDH-related haemolysin are the two key virulence factors in pathogenicity islands harbour the genes encoding these two haemolysins. The serotyping of is based on the combination of O and K antigens. Frequent recombination has been observed in , including in the genomic regions encoding the O and K antigens. serotype O4:K12 has caused gastroenteritis outbreaks in the USA and Spain. Recently, outbreaks caused by this serotype of have been reported in China. However, the relationships among this serotype of strains isolated in different regions have not been addressed. Here, we investigated the genome variation of the serotype O4:K12 using the whole-genome sequences of 29 isolates. We determined five distinct lineages in this strain collection. We observed frequent recombination among different lineages. In contrast, little recombination was observed within each individual lineage. We showed that the lineage of this serotype of isolated in America was different from those isolated in Asia and identified genes that exclusively existed in the strains isolated in America. Pan-genome analysis showed that strain-specific and cluster-specific genes were mostly located in the genomic islands. Pan-genome analysis also showed that the vast majority of the accessory genes in the O4:K12 serotype of were acquired from within the genus . Hence, we have shown that multiple distinct lineages exist in serotype O4:K12 and have provided more evidence about the gene segregation found in isolated in different continents.