The term Bacillus, although it designates a certain genus, is still often used synonymously for any “aerobic endospore-forming bacteria” (AEFB). Bergey's Manual of Systematic Bacteriology (Sneath et al., 1986) listed four genera and 44 species; however, taxonomical development since then has shown that at least 59 genera of aerobic spore-forming organisms with more than 419 species can be distinguished (Table 4.1) and therefore the expression “Bacillus and related genera” is more appropriate (Figure 4.1).

Validly published names of genera and species affiliated to AEFB are accessible via the internet, showing the list of prokaryotic names with standing in nomenclature (http://www.bacterio.cict.fr/).