Bacillus is a genus of Gram-positive, rod-shaped bacteria and a member of the phylum Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species. Under stressful environmental conditions, the cells produce oval endospores that can stay dormant for extended periods. These characteristics originally defined the genus, but not all such species are closely related, and many have been moved to other genera.
Many Bacillus species are able to secrete large quantities of enzymes. Bacillus amyloliquefaciens is the source of a natural antibiotic protein barnase (a ribonuclease), alpha amylase used in starch hydrolysis, the protease subtilisin used with detergents, and the BamH1 restriction enzyme used in DNA research.
Use as model organism
Bacillus subtilis is one of the best understood prokaryotes, in terms of molecular biology and cell biology. Its superb genetic amenability and relatively large size have provided the powerful tools required to investigate a bacterium from all possible aspects. Recent improvements in fluorescence microscopy techniques have provided novel and amazing insight into the dynamic structure of a single cell organism. Research on Bacillus subtilis has been at the forefront of bacterial molecular biology and cytology, and the organism is a model for differentiation, gene/protein regulation, and cell cycle events in bacteria.
Two Bacillus species are considered medically significant: B. anthracis, which causes anthrax, and B. cereus, which causes a foodborne illness similar to that of Staphylococcus. A third species, B. thuringiensis, is an important insect pathogen, and is sometimes used to control insect pests. The type species is B. subtilis, an important model organism. It is also a notable food spoiler, causing ropiness in bread and related food. Some environmental and commercial strains B. coagulans may play a role in food spoilage of highly acidic, tomato based products.
An easy way to isolate Bacillus is by placing nonsterile soil in a test tube with water, shaking, placing in melted mannitol salt agar, and incubating at room temperature for at least a day. Colonies are usually large, spreading and irregularly-shaped. Under the microscope, the Bacillus cells appear as rods, and a substantial portion usually contain an oval endospore at one end, making it bulge.
The cell wall of Bacillus is a structure on the outside of the cell that forms the second barrier between the bacterium and the environment, and at the same time maintains the rod shape and withstands the pressure generated by the cell's turgor. The cell wall is composed of teichoic and teichuronic acids. B. subtilis is the first bacterium for which the role of an actin-like cytoskeleton in cell shape determination and peptidoglycan synthesis was identified, and for which the entire set of peptidoglycan-synthesizing enzymes was localised. The role of the cytoskeleton in shape generation and maintenance is important.
The genus Bacillus was coined in 1835 by Christian Gottfried Ehrenberg (who coined the genus Bacterium seven years prior) to contain rod-shaped bacteria, later amended by Ferdinand Cohn to spore-forming, Gram-positive/variable, rod-shaped bacteria. Like other genera associated with the early history of microbiology, such as Pseudomonas or Vibrio, members of the Bacillus genus (266 species) are found ubiquitously, and it is one of the genera with the largest 16S diversity and environmental diversity.
Several studies have tried to reconstruct the phylogeny of the genus. The Bacillus-specific study with the most diversity covered is by Xu and Cote' using 16S and the ITS region, where they divide the genus into 10 groups, which includes the nested genera Paenibacillus, Brevibacillus, Geobacillus, Marinibacillus and Virgibacillus. However, the tree  constructed by the living tree project, a collaboration between ARB-Silva and LPSN where a 16S (and 23S if available) tree of all validated species was constructed, the genus Bacillus contains a very large number of nested taxa and majorly in both 16S and 23S it is paraphyletic to Lactobacillales (Lactobacillus, Streptococcus, Staphylococcus, Listeria, etc.), due to Bacillus coahuilensis and others. A gene concatenation study found similar results to Xu and Cote', but with a much more limited number of species in terms of groups, but used Listeria as an outgroup, so in light of the ARB tree, it may be "inside-out".
One clade, formed by B. anthracis, B. cereus, B. mycoides, B. pseudomycoides, B. thuringiensis and B. weihenstephanensis under current classification standards, should be a single species (within 97% 16S identity), but due to medical reasons, they are considered separate species, an issue also present for four species of Shigella and Escherichia coli.
|Phylogeny of the genus Bacillus according to |
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- Bacillus Anthracis
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