Bacillus anthracis, the etiologic agent of anthrax, is a close relative of B. cereus, a soil organism and known opportunistic pathogen that causes a variety of human infections [1]. B. anthracis is very similar to B. cereus and B. thuringiensis except that all confirmed samples of B. anthracis suggest that it is a monophyletic clone derived from the B. cereus and B. thuringiensis clade. The major distinguishing feature of B. anthracis is the presence of two large virulence plasmids, pXO1 and pXO2, that harbor the tripartite toxin complex [2] and the genes responsible for the synthesis of a poly-γ-d-glutamic acid capsule [3,4], respectively. Although virulence factors associated with pathogenic B. cereus isolates are not understood, large plasmids are known to be associated with many of the soil and pathogenic isolates [5] and are likely to impart advantageous phenotypes that promote opportunistic pathogenic properties and/or growth in soil. Recent studies now demonstrate that the “genetic backbones” for both the pXO1 and pXO2 plasmids are not restricted to B. anthracis but rather can be found in related B. cereus and B. thuringiensis isolates as well [6–9].