Interestingly, most of the above species share a remarkably high level of 16S rRNA gene sequence similarity to B. subtilis (often 99 % or greater) even though DNA–DNA hybridization values with the latter fall below 70 %. Moreover, very few phenotypic or biochemical characteristics can be found that differentiate these species from B. subtilis. This high degree of phenotypic and biochemical similarity prompted Gordon et al. (1973) to characterize species closely related to B. subtilis as members of the ‘subtilis-group’ or ‘subtilis-spectrum’. Herein, we refer to this assemblage as the B. subtilis species complex.