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In 1872, Ferdinand Cohn, a contemporary of Robert Koch, recognized and named the bacterium Bacillus subtilis. The organism is Gram-positive, capable of growth in the presence of oxygen, and forms a unique type of resting cell called an endospore. The organism represented what was to become a large and diverse genus of bacteria named Bacillus, in the Family Bacillaceae.

Koch relied on Cohn's observations in his classic work (1876), The etiology of anthrax based on the life history of Bacillus anthracis, which provided the first proof that a specific microorganism could cause a specific disease.

Robert Koch's original photomicrographs of Bacillus anthracis. In 1876, Koch established by careful microscopy that the bacterium was always present in the blood of animals that died of anthrax. He took a small amount of blood from such an animal and injected it into a healthy mouse, which subsequently became diseased and died. He was able to recover the original anthrax organism from the dead mouse, demonstrating for the first time that a specific bacterium is the cause of a specific disease. 

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".

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.

Bacillus coagulans. Gram stain. CDC. Gram-positive or Gram-negative? The cell wall structure of endospore-forming bacteria is consistent with that of Gram-positive bacteria, and young cultures stain as expected. However, many sporeformers rapidly become Gram-negative when entering the stationary phase of growth.

Classification and Phylogeny

Early attempts at classification of Bacillus species were based on two characteristics: aerobic growth and endospore formation.  This resulted in tethering together many bacteria possessing different kinds of physiology and occupying a variety of habitats. Hence, the heterogeneity in physiology, ecology, and genetics, made it difficult to categorize the genus Bacillus or to make generalizations about it.

In Bergey's Manual of Systematic Bacteriology (1st ed. 1986), the G+C content of known species of Bacillus ranges from 32 to 69%.  This observation, as well as DNA hybridization tests, revealed the genetic heterogeneity of the genus. Not only was there variation from species to species, but there were sometimes profound differences in G+C content within strains of a species. For example, the G+C content of the Bacillus megaterium group ranged from 36 to 45%.

In Bergey's Manual of Systematic Bacteriology (2nd ed. 2004), phylogenetic classification schemes landed the two most prominent types of endospore-forming bacteria, clostridia and bacilli, in two different Classes of Firmicutes, Clostridia and Bacilli.  Clostridia includes the Order Clostridiales and Family Clostridiaceae with 11 genera including, Clostridium. Bacilli includes the Order Bacillales and the Family Bacillaceae. In this family there 37 new genera on the level with Bacillus. This explains the heterogeneity in G+C content observed in the 1986 genus Bacillus.

The phylogenetic approach to Bacillus taxonomy has been accomplished largely by analysis of 16S rRNA molecules by oligonucleotide sequencing. This technique, of course, also reveals phylogenetic relationships. Surprisingly, Bacillus species showed a kinship with certain nonsporeforming species, including Enterococcus, Lactobacillus, and Streptococcus at the Order level, and Listeria and Staphylococcus at the Family level. Otherwise, some former members of the genus Bacillus were gathered into new Families, including Acyclobacillaceae, Paenibacillaceae and Planococcaceae, now on the level with Bacillaceae. Most of the bacteria discussed in this article come from one of these four Families. Their taxonomic hierarchy (Bergey's 2004) is Kingdom: Bacteria; Phylum: Firmicutes; Class: Bacilli; Order: Bacillales; Family: Acyclobacillaceae (genus: Acyclobacillus); Family: Bacillaceae (genus: Bacillus, Geobacillus); Family: Paenibacillaceae (genus: Paenibacillus, Brevibacillus); Family: Planococcaceae (genus: Sporosarcina).

Notable former members of the genus Bacillus that have been moved to new families and/or genera are given in the table below.