Some Bacillus thuringiensis strains share the same rpoB nucleotide polymorphisms of Bacillus anthracis

Identification of Bacillus anthracis is considerably difficult because of its very high phenotypic and genotypic similarity to other members of the Bacillus cereus group. Differentiation methods based on morphological and phenotypic characteristics are time-consuming, and false results may be obtained for atypical strains. On the other hand molecular discrimination based on the presence of two B. anthracis virulence plasmids, pXO1 and pXO2, is not sufficient because plasmids can be lost or transferred to other bacilli. Therefore, several chromosomal markers have been investigated and applied (1, 8). In 2001 Qi et al. (7) described single nucleotide polymorphisms (SNPs) in the rpoB gene and their usefulness for B. anthracis identification. Since then, several articles describing various molecular methods for rpoB sequence-based detection of B. anthracis have been published (for example see references 2 and 9). We conducted studies of single-strand conformation polymorphisms (SSCPs) of the rpoB gene in a large collection of B. cereus group strains. Surprisingly, we found that the nucleotide sequence of the rpoB gene fragment containing the marker SNPs of two reference strains of Bacillus thuringiensis was identical to that of the homologous region in B. anthracis. Therefore, rpoB gene-based tools could not distinguish these strains from B. anthracis, thus resulting in false-positive anthrax identification.