Evaluation of Bacillus strains as model systems for the work on Bacillus anthracis spores

Available strain collections of Bacillus anthracis and Bacillus cereus were screened for B. cereus strains sharing major genotypic characteristics with B. anthracis. Based on the comparison of partial spoIIIAB sequences, whole genome sequences and MLST, a strain set representing different lineages including candidate model strains for B. anthracis was compiled. Spores from the selected strain set and two B. anthracis strains were prepared according to a newly optimized protocol transferable to biosafety level-3 (BSL3) conditions and phenotypic characteristics including scanning electron microscopy (SEM), heat inactivation, and germination were evaluated. Two B. cereus isolates were identified that were genetically related to B. anthracis and showed high similarity to B. anthracis spores in their heat inactivation profile and their response to the germinants l-alanine and inosine. In addition, these isolates were also mimicking B. anthracis on modified PLET, a selective plating medium for B. anthracis, and shared various other biochemical characteristics with B. anthracis. Therefore these two strains are not only appropriate models for B. anthracis in experiments based on spore characteristics but also in trials working with plating media. These two strains are now used within the BIOTRACER consortium as validated models for B. anthracis and will facilitate the development and optimization of tracing and detection systems for B. anthracis in the food and feed chain.