ABSTRACTBacillus anthracisplasmids 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 theBacillus cereussensu latogroup remain unclear. To gain new insights into these issues, we sequenced 412B. anthracisstrains representing the total phylogenetic and ecological diversity of the species. Our results revealed thatB. anthracisgenomes 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 (P< 0.05 [Welch two-sample t test]). Comparisons againstB. cereusgenomes carrying complete or partial pXO1-like and pXO2-like plasmids showed that the plasmid-based phylogeny recapitulated that of the main chromosome, indicating limited plasmid horizontal transfer between or within these species. Comparisons of gene content revealed a closed pXO1 and pXO2 pangenome; e.g., plasmids encode <8 unique genes, on average, and a single large fragment deletion of pXO1 in oneB. anthracisstrain (2000031682) was detected. Collectively, our results provide a more complete view of the genomic diversity ofB. anthracisplasmids, their copy number variation, and the virulence potential of otherBacillusspecies carrying pXO1/pXO2-like plasmids.IMPORTANCEBacillus anthracismicroorganisms are of historical and epidemiological importance and are among the most homogenous bacterial groups known, even though theB. anthracisgenome is rich in mobile elements. Mobile elements can trigger the diversification of lineages; therefore, characterizing the extent of genomic variation in a large collection of strains is critical for a complete understanding of the diversity and evolution of the species. Here, we sequenced a large collection ofB. anthracisstrains (>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 betweenB. anthracisand pathogenicB. cereusisolates but rather predominantly vertical descent. These findings advance our understanding of the biology and pathogenomic evolution ofB. anthracisand its plasmids.
Industrial Relevance of Chromosomal Copy Number Variation in Saccharomyces Yeasts
