Genomic insights into the taxonomic status of the three subspecies of Bacillus subtilis

Bacillus subtilis contains three subspecies, i.e., subspecies subtilis, spizizenii, and inaquosorum. As these subspecies are phenotypically indistinguishable, their differentiation has relied on phylogenetic analysis of multiple protein-coding gene sequences. B. subtilis subsp. inaquosorum is a recently proposed taxon that encompasses strain KCTC 13429^T and related strains, which were previously classified as members of subspecies spizizenii. However, DNA-DNA hybridization (DDH) values among the three subspecies raised a question as to their independence. Thus, we evaluated the taxonomic status of subspecies inaquosorum using genome-based comparative analysis. In contrast to the previous experimental values of DDH, the inter-genomic relatedness inferred by average nucleotide identity (ANI) values indicated that subspecies inaquosorum and spizizenii were sufficiently different from subspecies subtilis and hence raised the possibility that the former two could be classified as separate species from B. subtilis. The genome-based tree also supported the separation of the two subspecies from B. subtilis. The exclusive presence of a subtilin synthesis system in subspecies spizizenii was a remarkable genetic characteristic that could even distinguish subspecies spizizenii from subspecies inaquosorum in addition to the low ANI values (<95%). Conclusively, the genome-based data obtained in this study demonstrated that subspecies inaquosorum and spizizenii are clearly distinguished from subspecies subtilis, and raises the possibility that these two subspecies could be classified as separate species from B. subtilis. In addition, the low ANI values between subspecies inaquosorum and spizizenii and the exclusive presence of subtilin synthesis genes in subspecies spizizenii also suggest circumscription of these two subspecies at the species level.