Virulence Megaplasmids in Bacillus anthracis and Their Relatives in the Bacillus cereus Group

Bacillus anthracis is the etiological agent of anthrax. In the 1980s, a link was discovered between plasmids and major virulence factors of B. anthracis. Indeed, the three toxin components are encoded by pXO1, a 181-kb plasmid, and the poly-glutamate capsule biosynthetic operon is carried by pXO2, a 97-kb plas-mid. The functions encoded by a few other genes were described after screening or selecting for specific phenotypes, such as regulation of virulence factor synthesis. Despite a renewal of interest in B. anthracis at the end of the twentieth century, which prompted further research and led to complete sequencing of both plas-mids in 1999, only few genes have been fully characterized. These include genes involved in replication, sporulation, and germination. Yet, 40% of the open reading frames (ORFs) are of unknown function, and for most of the others a function has only been predicted in silico. B. anthracis is, on a genetic basis, a Bacillus cereus. B. cereus strains also harbor megaplasmids, some sharing core sequences with pXO1 and pXO2. These plasmids also encode virulence factors or specific environmental adaptive pathways.