Bacillus
Genus of bacteria
From Wikipedia, the free encyclopedia
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]
| 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] | |
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]
Phylogeny
A core-proteome analysis of more than 1,100 genomes examined relationships within the B. subtilis and B. cereus groups using 114 conserved proteins and genome-wide nucleotide similarity.[11]
The Genome Taxonomy Database (GTDB) infers its bacterial reference tree from 120 single-copy marker proteins.[12] In release R11-RS232, species retained by LPSN in Bacillus are distributed among several genome-defined GTDB genera. The two largest clusters, Bacillus and Bacillus_A, correspond broadly to the B. subtilis and B. cereus groups, respectively.[13][14] The suffix in Bacillus_A is a GTDB placeholder and is not part of a validly published genus name. The two pruned subtrees below show selected species with validly published and correct names under LPSN; they are not a comprehensive tree of all species assigned to Bacillus by LPSN. Unnamed genome clusters were excluded, and where GTDB divided a nomenclatural species among multiple clusters, one cluster was shown.[1][14]
| GTDB genus Bacillus (B. subtilis group) | GTDB genus Bacillus_A (B. cereus group) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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]
Cell envelope and shape
The cell wall forms a barrier between the cell and its environment, maintains the rod shape, and withstands the pressure generated by cellular turgor. In B. subtilis, the wall contains peptidoglycan and anionic polymers, including teichoic acids. This species was also among the first bacteria in which an actin-like cytoskeleton was shown to participate in cell-shape determination and peptidoglycan synthesis.[15]
Endospore formation
Under nutrient limitation or other adverse conditions, many Bacillus cells initiate sporulation. Following asymmetric cell division, the larger mother cell engulfs the smaller forespore, which matures into an endospore and is eventually released when the mother cell lyses. One endospore is ordinarily formed per vegetative cell, and it is a non-reproductive survival structure rather than an offspring.[16]
Endospores can remain dormant for long periods and are substantially more resistant than vegetative cells to heat, cold, desiccation, radiation, and many chemical agents. The degree of resistance varies among species, strains, environmental conditions, and treatments.[17] Their resistance makes spores difficult to eliminate from foods, medical materials, and pharmaceutical or industrial environments, where they can be persistent contaminants.[2]
A laboratory study reported that dry spores of B. amyloliquefaciens remained viable after exposure to 420 °C under the experimental conditions.[18] The experiment concerned survival of dry, dormant spores rather than vegetative growth at that temperature.
B. anthracis ordinarily requires oxygen to sporulate, so spores are generally produced after vegetative cells leave an infected host. This constraint has important consequences for the environmental persistence and control of anthrax.[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
Enzymes and biotechnology
Many Bacillus strains secrete large quantities of enzymes. This ability has made B. subtilis, B. licheniformis, and related species important industrial production microorganisms. Commercial products include amylases used in starch processing and proteases, including subtilisins, used in detergents.[6]
The capacity of selected strains to secrete proteins at high concentrations has also led to their use as hosts for heterologous protein production. Product degradation by host proteases and inefficient folding or export can limit yields, so strain engineering and protein engineering are used to improve secretion. Bacillus strains have additionally been developed to produce compounds such as riboflavin, nucleotides, and poly-γ-glutamic acid.[6]
Food fermentation
Strains of B. subtilis are used to ferment soybeans in the production of nattō.[24] Conversely, the persistence and heat resistance of endospores make some members of the genus important spoilage organisms in food processing.[2]
Biological pest control
B. thuringiensis produces parasporal crystals containing Cry and Cyt proteins that are toxic to particular groups of insects and other invertebrates. Preparations of the bacterium are used as biological insecticides, and genes encoding some of its insecticidal proteins have been introduced into insect-resistant crop plants.[5][25]
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]
