The Study of Diversity in Sporulation among Closely Genetically Related Bacillus cereus Strains.

Bacillus cereus is an important foodborne pathogenic bacterium. Although several B. cereus strains have been isolated from the environment, the differences among these strains with respect to spore formation ability and cell morphology need clarification. In this study, a phylogenetic tree was constructed based on the 16S rRNA gene sequences of nine strains of B. cereus. Spore formation and morphology of these nine strains were compared using both phase-contrast and fluorescence microscopy to create an index of the designated sporulation stages. Additionally, to investigate the efficiency of heat-resistant spore formation. Phylogenetic analysis revealed that five strains (ATCC 14579^T, NBRC 3457, NBRC 3514, NBRC 3836, and NBRC 13597) clustered together and the remaining four (ATCC 10987, NBRC 3003, NBRC 13494, and NBRC 13690) were genetically distinct from each other. Phase-contrast microscopy revealed significant differences in the sporulation stages among the nine strains. Furthermore, the efficiency of heat-resistant spore formation also differed, even among genetically related strains. In conclusion, a variety of cell morphologies during sporulation were observed among the nine B. cereus strains. We propose a designation of sporulation stages in B. cereus ATCC 14579^T, which may be used as an index for evaluating the sporulation progress of B. cereus.