Evolution of the Bacillus cereus Group

Abstract

The Bacillus cereus group contains six approved bacterial species-Bacillus thuringiensis, Bacillus cereus (sensu stricto), Bacillus anthracis, Bacillus weihenstephanensis, Bacillus mycoides, and Bacillus pseudomycoides. In addition, thermotolerant B. cereus var. cytotoxis could constitute a member of a novel species in the group. Historically, Bacillus thuringiensis has been viewed as a species separate from the other members of the group, mainly based on its entomopathogenic properties and production of insecticidal crystal toxins. However, following the sequencing of more than 85 strains from the B. cereus group, whole genome phylogenies and MLST analyses of more than 1400 strains based on chromosomal markers clearly show that the Bacillus cereus group exhibits a complex population structure, in which strains of B. thuringiensis are strongly intermixed with other B. cereus group species. From the analysis of genome sequence data, it is clear that the only principal factor differentiating B. thuringiensis from B. cereus is the presence of insecticidal crystal toxin gene(s), which in the vast majority of cases are carried on plasmid elements that are frequently conjugative or mobilizable, and which occasionally could be lost by the bacterium. Also, strikingly, some B. thuringiensis strains are among the closest relatives to the B. anthracis clonal complex. Chromosomal factors potentially involved in stabilizing long-term carriage of plasmids in B. thuringiensis strains, have not yet been elucidated.