Bacillus
Gram-stained cells of Bacillus subtilis
Scientific classification
Domain:	Bacteria
Kingdom:	Bacillati
Phylum:	Bacillota
Class:	Bacilli
Order:	Caryophanales
Family:	Bacillaceae
Genus:	Bacillus Cohn 1872 (Approved Lists 1980)[1]
Type species
Bacillus subtilis (Ehrenberg 1835) Cohn 1872 (Approved Lists 1980)
Species[1]
Selected species

Bacillus is a genus of mostly Gram-stain-positive, rod-shaped bacteria in the family Bacillaceae. Its type species is Bacillus subtilis.^[1] Most members are aerobic or facultatively anaerobic, and many can form resistant endospores. Some species or older cultures may be Gram-variable.^[2]

Members of the genus occur in soil, water, sediments, foods, and in association with plants and animals. Several species have medical, agricultural, food, or industrial importance. Bacillus anthracis causes anthrax, Bacillus cereus can cause food poisoning and other infections, Bacillus thuringiensis produces insecticidal proteins, and B. subtilis is widely used as a model organism and in biotechnology.^[3][4][5][6]

Taxonomy and nomenclature

The name is derived from the Latin masculine noun bacillus, meaning a small staff, wand, or rod.^[1] The genus is attributed to Ferdinand Cohn, who placed the species originally described by Christian Gottfried Ehrenberg as Vibrio subtilis in Bacillus.^[1] The term "bacillus" is also used informally for any rod-shaped bacterium, while Bacilli is the name of the bacterial class that contains the genus.

The historical circumscription of Bacillus included numerous distantly related endospore-forming bacteria. Analyses of small-subunit ribosomal RNA sequences demonstrated extensive phylogenetic heterogeneity within the traditional genus.^[7] Comparative genomic analyses subsequently confirmed that the broadly defined genus was polyphyletic.^[8]

A phylogenomic revision transferred six major clades to the genera Peribacillus, Cytobacillus, Mesobacillus, Neobacillus, Metabacillus, and Alkalihalobacillus.^[9] A further revision transferred additional clades to 17 other genera and emended Bacillus primarily around the B. subtilis and B. cereus groups.^[10] The circumscription continues to change as species and genera are reassessed.^[1]

Morphology and physiology

Vegetative cells generally occur as straight or slightly curved rods, singly or in chains. Young cultures usually stain Gram-positive, although cells of some species may become Gram-variable or appear Gram-negative as cultures age. Most members are catalase-positive and grow aerobically or facultatively anaerobically, but physiological characteristics vary across the genus.^[2]

Isolation and identification

Established methods for isolating endospore-forming bacteria from soil include suspending the sample in water, applying a heat shock to kill many vegetative cells while leaving viable spores, and culturing the treated sample on agar.^[19] Hot-air drying and similar treatments can also enrich environmental samples for spore-forming isolates.

Many cultured soil isolates form large, spreading, or irregular colonies, and microscopy may reveal rod-shaped cells containing oval endospores. These features are not universal or specific to Bacillus. Morphology and endospore formation alone are therefore insufficient for reliable identification; phenotypic tests are now supplemented by sequence- or genome-based methods.^[9][10]

Ecology

Bacillus species are widely distributed in terrestrial and aquatic environments. Endospore formation facilitates their survival during nutrient limitation and exposure to environmental stress.^[2][17] Some species occur on plant surfaces, in the rhizosphere, or as endophytes. Selected strains can promote plant growth, increase nutrient availability, or suppress plant pathogens, although performance depends on the strain and environmental conditions.^[20]

Some strains synthesize and secrete lipopeptides, including surfactins and mycosubtilins, which can contribute to antimicrobial activity and interactions with plants or other microorganisms.^[21][22]

Members of the genus have also been isolated from marine sponges. A study of bacteria associated with sponges from the Bay of Bengal recovered several B. subtilis strains with antimicrobial activity, including strains investigated for use as probiotics in aquaculture.^[23]

Medical importance

B. anthracis is the causative agent of anthrax. In a mammalian host, vegetative cells produce a poly-D-glutamic acid capsule that helps protect them from phagocytosis.^[3][2]

B. cereus can cause two main forms of food poisoning: an emetic syndrome associated with the toxin cereulide, and a diarrheal syndrome associated with enterotoxins. It can also cause local and systemic infections.^[4] Other species are occasionally recovered as opportunistic pathogens, although environmental Bacillus can also represent specimen contamination; their clinical significance depends on the species, isolation site, and patient context.^[2]

Industrial and agricultural uses

Model organism

B. subtilis is one of the best-understood Gram-positive bacteria. Its genetic tractability and comparatively large cells have made it a model for research on endospore formation, cell differentiation, chromosome organization, gene regulation, protein secretion, and the bacterial cell cycle. Fluorescence-microscopy methods have also been used extensively to examine the dynamic organization of individual B. subtilis cells.^[26]

Selected species

As of August 2026, the List of Prokaryotic names with Standing in Nomenclature recorded 125 child taxa of Bacillus with validly published and correct names.^[1] Species discussed in this article include:

Many species formerly placed in Bacillus have been transferred to other genera, and older literature and databases may therefore use different combinations.^[9][10]

See also

• Bacillus (shape)
• List of bacteria genera
• List of bacterial orders

References

External links

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