Of 87 strains previously identified as Bacillus brevis Migula 1900, 58 had G + C contents of 47.0 to 51.9 mol%, a range that included the G + C content (48.7 mol%) of the type strain. The G + C contents for three other groups consisting of 5, 7, and 17 strains were 37.0 to 41.9, 42.0 to 46.9, and 52.0 mol% or higher, respectively. DNA reassociation studies showed that 25 of the 58 strains with G + C contents of 47.0 to 51.9 mol% were closely related genetically to the type strain and to each other. For the most part, this genetically related group was phenotypically homogeneous; variations in the fermentation of mannitol and mannose were observed. My results strongly suggest that many of the strains were misclassified as B. brevis. Consequently, much of the phenotypic heterogeneity of the species B. brevis Migula 1900 is not due to variations exhibited by genetically related organisms, but is the result of variability introduced by the presence of genetically unrelated strains.

16S rRNA gene sequences of the type strains of 11 species belonging to the Bacillus brevis and Bacillus aneurinolyticus groups were determined. On the basis of the results of gene sequence analyses, these species were separated into two clusters. The B. brevis cluster included 10 species, namely, Bacillus brevis, Bacillus agri, Bacillus centrosporus, Bacillus choshinensis, Bacillus parabrevis, Bacillus reuszeri, Bacillus formosus, Bacillus borstelensis, Bacillus laterosporus, and Bacillus thermoruber. Bacillus aneurinolyticus and Bacillus migulanus belonged to the B. aneurinolyticus cluster. Moreover, the two clusters were phylogenetically distinct from other Bacillus, Amphibacillus, Sporolactobacillus, Paenibacillus, and Alicyclobacillus species. On the basis of our data, we propose reclassification of the B. brevis cluster as Brevibacillus gen. nov. and reclassification of the B. aneurinolyticus cluster as Aneurinibacillus gen. nov. By using 16S rRNA gene sequence alignments, two specific PCR amplification primers were designed for differentiating the two new genera from each other and from other aerobic, endospore-forming organisms.