Loss and Gain in the Evolution of the Salmonella enterica Serovar Gallinarum Biovar Pullorum Genome

Salmonella enterica subspecies enterica serovar Gallinarum biovar Pullorum (S Pullorum) is the etiological agent of pullorum disease, causing white diarrhea with high mortality in chickens. There are many unsolved issues surrounding the epidemiology of S Pullorum, including its origin and transmission history as well as the discordance between its phenotypic heterogeneity and genetic monomorphism. In this paper, we report the results of whole-genome sequencing of a panel of 97 S Pullorum strains isolated between 1962 and 2014 from four countries across three continents. We utilized 6,795 core genome single nucleotide polymorphisms (SNPs) to reconstruct a phylogenetic tree within a spatiotemporal Bayesian framework, estimating that the most recent common ancestor of S Pullorum emerged in ∼914 CE (95% confidence interval [95%CI], 565 to 1273 CE). The extant S Pullorum strains can be divided into four distinct lineages, each of which is significantly associated with geographical distribution. The intercontinental transmissions of lineages III and IV can be traced to the mid-19th century and are probably related to the "Hen Fever" prevalent at that time. Further genomic analysis indicated that the loss or pseudogenization of functional genes involved in metabolism and virulence in S Pullorum has been ongoing since before and after divergence from the ancestor. In contrast, multiple prophages and plasmids have been acquired by S Pullorum, and these have endowed it with new characteristics, especially the multidrug resistance conferred by two large plasmids in lineage I. The results of this study provide insight into the evolution of S Pullorum and prove the efficiency of whole-genome sequencing in epidemiological surveillance of pullorum disease.IMPORTANCE Pullorum disease, an acute poultry septicemia caused by Salmonella Gallinarum biovar Pullorum, is fatal for young chickens and is a heavy burden on poultry industry. The pathogen is rare in most developed countries but still extremely difficult to eliminate in China. Efficient epidemiological surveillance necessitates clarifying the origin of the isolates from different regions and their phylogenic relationships. Genomic epidemiological analysis of 97 S Pullorum strains was carried out to reconstruct the phylogeny and transmission history of S Pullorum. Further analysis demonstrated that functional gene loss and acquisition occurred simultaneously throughout the evolution of S Pullorum, both of which reflected adaptation to the changing environment. The result of our study will be helpful in surveillance and prevention of pullorum disease.