Deoxyribonucleic Acid Homology and Taxonomy of the Genus Bacillus

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Abstract
The taxonomic relationships among 56 strains of 16 species of the genus Bacillus were studied by deoxyribonucleic acid (DNA)-DNA hybridization. In general, no significant DNA homology was detected between two strains of different species, except for a group of species consisting of B. subtilis, B. amyloliquefaciens, B. licheniformis, and B. pumilus and for another group of species including B. cereus and B. thuringiensis. Species of the former group were related, but they were independent of each other as their DNA homologies were 19% or less. The DNA homology indexes of three strains of B. thuringiensis to B. cereus T, so far tested, showed high DNA homologies (54 to 80%). This fact indicates that B. thuringiensis should be an identical species to B. cereus. The intraspecific DNA homology indexes of 16 strains of B. pumilus were 51% or more to strain IFO 12092 as the standard, and those of 10 strains of B. coagulans were 76 to 113% to strain ATCC 7050 as the standard. Thus, the species identification of B. pumilus and B. coagulans by the conventional taxonomic method was well in accord with the DNA homology data. On the other hand, significant heterogeneities were suggested among the strains of B. circulans and among those of B. sphaericus by the DNA homology data of the three and five strains so far tested, respectively. Although B. lentus was described to be closely related to B. firmus in Bergey's Manual (8th ed., 1974), the interspecific DNA homology index between these two species was 3%. It was concluded that these two species are independent.