mSystems

Bacillus anthracis plasmids pXO1 and pXO2 carry the main virulence factors responsible for anthrax. However, the extent of copy number variation within the species and how the plasmids are related to pXO1/pXO2-like plasmids in other species of the Bacillus cereus | group remain unclear. To gain new insights into these issues, we sequenced 412 B.\ua0anthracis strains representing the total phylogenetic and ecological diversity of the species. Our results revealed that B.\ua0anthracis genomes carried, on average, 3.86 and 2.29 copies of pXO1 and pXO2, respectively, and also revealed a positive linear correlation between the copy numbers of pXO1 and pXO2. No correlation between the plasmid copy number and the phylogenetic relatedness of the strains was observed. However, genomes of strains isolated from animal tissues generally maintained a higher plasmid copy number than genomes of strains from environmental sources (| 400) that were recovered from human, animal, and environmental sources around the world. Our results confirmed the remarkable stability of gene content and synteny of the anthrax plasmids and revealed no signal of plasmid exchange between B.\ua0anthracis and pathogenic B.\ua0cereus isolates but rather predominantly vertical descent. These findings advance our understanding of the biology and pathogenomic evolution of B.\ua0anthracis and its plasmids.30116789PMC609398