New method for differentiating members of the Bacillus cereus group: collaborative study.

A collaborative study was conducted of a new method for differentiating members of the Bacillus cereus group. Using the new method, each of 14 collaborators identified 8 Bacillus cultures, which represented 3 biotypes of the B. cereus group. Each culture was tested for motility, hemolytic activity on trypticase-soy-sheep blood agar, and rhizoid growth on nutrient agar; carbol-fuchsin stain was used to determine the presence of protein toxin crystals. Cultures were identified as B. cereus (biotype 1), B. cereus var. mycoides (biotype 2), or B. thuringiensis (biotype 3). All collaborators correctly identified the unknown cultures and classified them correctly as to biotype. There were no statistically significant differences in the identification rates among the different laboratories. Additional tests by one participant on 5 strains of Bacillus anthracis showed that the new method is also adequate for differentiating B. anthracis from typically reacting strains of B. cereus. The method has been adopted interim official first action.