The genus Bacillus remained intact until 2004, when it was split into several families and genera of endospore-forming bacteria, justifiable on the basis of ssRNA analysis. In order to accommodate former members of the genus Bacillus covered in this chapter, its title has been changed to "Gram-positive aerobic or facultative endospore-forming bacteria".
The unifying characteristic of these bacteria is that they are Gram-positive, form endospores, and grow in the presence of O2. The trivial name assigned to them is aerobic sporeformers.
The ubiquity and diversity of these bacteria in nature, the unusual resistance of their endospores to chemical and physical agents, the developmental cycle of endospore formation, the production of antibiotics, the toxicity of their spores and protein crystals for many insects, and the pathogen Bacillus anthracis, have attracted ongoing interest in these bacteria since and Cohn and Koch's discoveries in the 1870s.
There is great diversity of physiology among the aerobic sporeformers, not surprising considering their recently-discovered phylogenetic diversity. Their collective features include degradation of most all substrates derived from plant and animal sources, including cellulose, starch, pectin, proteins, agar, hydrocarbons, and others; antibiotic production; nitrification; denitrification; nitrogen fixation; facultative lithotrophy; autotrophy; acidophily; alkaliphily; psychrophily; thermophily; and parasitism. Endospore formation, universally found in the group, is thought to be a strategy for survival in the soil environment, wherein these bacteria predominate. Aerial distribution of the dormant spores probably explains the occurrence of aerobic sporeformers in most habitats examined