Basil Manning
Illustration of bacteria of the genus Bacillus

What is it Bacillus?

Bacillus is a genus of bacteria of the Firmicutes division characterized by being spore-forming under aerobic conditions. They are rod-shaped and are generally Gram positive (although some are Gram variable), with colonies that present organisms that stain pink and others that stain purple.

In these cases, researchers have found that the number of organisms that respond as Gram negative increases with the age of the colony due to a decrease in the thickness of the petidoglycan layer..

Bacteria of this genus can be strict aerobic or facultative anaerobic. Most are mobile due to the presence of flagella, however, there are also non-mobile representatives. They can be found in virtually any environment on earth, including extreme environments, from high altitudes to the seafloor..

Some species are of medical interest, as they can cause diseases such as anthrax (Bacillus anthracis) or foodborne illness (Bacillus cereus). Others are used to obtain antibiotics, enzymes, as probiotics or in fermentation processes and in agriculture..

Gender characteristics Bacillus

Bacillus subtilis culture. Taken and edited from: A doubt [CC BY-SA 4.0 (].

Endospore production

The main defining characteristic of the genus is the ability to produce endospores under aerobic conditions. These spores are characterized by being able to resist high temperatures, desiccation, the action of disinfectants, and even radiation.

Cellular wall

The cell wall is made up of several layers of peptidoglycans cross-linked between them, forming a strong scaffold that maintains the cell shape and contains teichoic and lipotheichoic acids..


They are rod-shaped, straight, or slightly curved and can be found singly, in pairs, and occasionally in chains. The vast majority of them are mobile due to the presence of peritrichous flagella, that is, they project in all directions. However, Bacillus anthracis lacks flagella.

Gram positive

Most of the bacteria of this genus are Gram positive, however, some are Gram variable, that is, they can be stained pink or purple. This is because the peptidoglycan layer can become less thick and complex as the bacteria age, being unable to retain the crystal violet when treated with alcohol..


They are ubiquitous and very resistant organisms, with species capable of resisting very high temperatures (thermophilic) or very low (psychrophilic), there are also species that can tolerate very acidic or very alkaline environments for other species.

Some species are strict aerobic, being unable to survive in anoxic conditions, while other species are facultative anaerobic.

Gender morphology Bacillus

Bacteria of the genus Bacillus They can be rod-shaped, straight or slightly curved, generally round-ended, even though some cells have been described as square (for example Bacillus cereus).

The cells have a diameter that varies from 0.4 to 1.8 microns and a length of 0.9 to 10.0 microns. The dimensions of the cells within each species and within each strain tend to have little variability.

Cells occur singly and in pairs, some in chains and occasionally in long filaments. Depending on the species, strain, and culture conditions, daughter cells can be separated.

Thus the culture appears to be composed of single cells and pairs of dividing cells when viewed by phase contrast microscopy. In other cases, the daughter cells may remain attached to each other, thus showing chains of cells..

The morphology of the spores is a taxonomic characteristic, although some variation may exist within particular strains. The most common spores are ellipsoidal or oval in shape, but shapes range from cylindrical to ellipsoidal, spherical, or irregularly shaped reminiscent of a kidney or banana..

Taxonomy and classification

Taxonomically, the genus Bacillus It is located in the Phylum Firmicutes, Class Bacilli, Order Bacillales, Family Bacillacea. This genus was used by Christian Gottfried Ehrenberg in 1835 to group rod-shaped bacteria.

Then Ferdinand Cohn, in 1872, redefined the group as spore-forming, heat-resistant, Gram-positive, and facultative aerobic or anaerobic bacteria. The type species of the genus is Bacillus subtilis.

Researchers determined in 1991 that the genus Bacillus, as it had been treated to date, it was polyphyletic. For which they carried out a rearrangement of the group Bacillus sensu lato, from which five new genera were extracted.

The genus is subdivided into two groups, on the one hand the group of B. subtilis and related species, while on the other is the group of B. cereus. The first of them groups organisms less than 1 μm in diameter, non-swollen sporangium and ellipsoidal spores..

The second group, on the other hand, contains species of more than 1 μm in diameter, with a non-swollen sporangium and elpsoidal spores..

Where is the gender Bacillus?

Bacteria of the genus Bacillus they have been isolated mostly from soil, water, food and clinical samples. But they have also been found in the most unusual environments, such as in oceanic sediments thousands of meters below sea level and in stratospheric air samples, in acidic geothermal pools, in highly alkaline groundwater and in terminal hypersaline lakes..

Others have been discovered in human-made niches, from Mexican tombs and decaying Roman wall paintings, to clean rooms in spacecraft assembly facilities..

Plants are also a rich source of new species of Bacillus, some endophytic and others associated with the rhizosphere.


Bacteria of the genus Bacillus have two forms of asexual reproduction: binary fission and sporulation.

Binary fission

Binary fission is a type of reproduction that occurs when the bacterium is in a favorable environment for its development and allows exponential growth. Binary fission involves mitotic division resulting in two identical daughter cells.

Reproduction of sporulation

This second type is also known for spore formation. It appears when there is some kind of stress in the colony. During sporulation an asymmetric cell division occurs, resulting in the formation of a larger cell (stem cell) and a smaller cell (prespore).

The prespore is encompassed by the stem cell and covered with several protective layers, which generates water loss and allows its maturity. After this, the lysis of the stem cell occurs and the release of the endospore, which can remain dormant until favorable conditions are re-established..

This endospore, as already pointed out before, is resistant to extreme temperatures, desiccation, the action of detergents and radiation, and is the main cause of the resistance of these bacteria and their ability to colonize any environment..


Most bacteria of the genus Bacillus They are found living in the soil and their nutrition is of the saprophytic type, that is, they feed on decomposing organic matter.

Other species make up the bacterial flora of animals. In these cases, they establish a symbiotic relationship in which they take advantage of the food ingested by their host and produce enzymes that help in the digestion of these foods..

Finally, some species can act as opportunistic parasites, feeding directly on their host..


Most species of Bacillus they are not pathogenic and have rarely been associated with disease in humans or other animals. Anthrax is the best known condition caused by this type of bacteria, although food poisoning and opportunistic infections caused by Bacillus cereus

Anthrax or anthrax

This disease is highly contagious and is caused by the bacteria Bacillus anthracis. It can affect different parts of the body and its pathogenicity depends on the affected tissue, the form of infection, and the time it takes from infection to treatment..

Bacillus anthracis culture, strain collected in Kaimedo, Tokyo, Japan, site of the 1993 bioterrorism incident. Taken and edited from: Centers for Disease Control, United States [Public domain].

The most aggressive form of the disease is pulmonary anthrax, which has a high mortality rate. It can also attack the skin (cutaneous anthrax) or the digestive system. Anthrax can attack any warm-blooded animal, including man.

The carcasses of animals killed by the disease, as well as land contaminated by the feces, or blood of infected animals constitute reservoirs of the disease.

Fried rice syndrome

It is a hemetic syndrome that appears mainly due to the consumption of poorly handled rice, although it can also appear due to the consumption of dairy products and other foods. The causative agent is bacteria Bacillus cereus.

This syndrome is characterized by the presence of nausea and vomiting, which appear 1 to 5 hours after eating contaminated food. Abdominal cramps may also occur, but diarrhea is rare..

It is a self-limited disease that generally lasts for no more than 48 hours, in which it is important to avoid dehydration, rest and avoid dairy consumption while the symptoms of the syndrome persist.


Endophthalmitis is an eye infection due to different pathogenic organisms. The inflammatory response of the infected organ can cause trauma to the same eye.

Different species of bacteria are related to this ailment, one of the most relevant is Bacillus cereus, whose infections cause lesions with reserved prognoses.

Treatment of the disease consists of giving the patient clindamycin or vancomycin, dexamethasone is also recommended. Sometimes the disease can compromise vision. In these cases, surgical removal of the vitreous humor is recommended..

Other diseases

Bacteria of the genus Bacillus They are related to different diseases that mainly affect people with compromised immune systems. These diseases include endocarditis, bacteremia, skin and musculoskeletal infections, as well as keratitis..

The species Bacillus megaterium, one of the largest species of bacteria that exists, can cause brain abscesses.

Bacillus megatierium. Taken and edited from: Alexastely [CC BY-SA 4.0 (].

Uses / applications


According to the World Health Organization, probiotics are live microorganisms that cause beneficial effects on the health of people or animals that consume them in adequate amounts in their diet.

Some species of Bacillus are used as probiotics, including B. coagulans. Among the benefits of ingesting this bacteria, researchers have suggested that it relieves the symptoms of irritable bowel syndrome. Also relieves symptoms of rheumatoid arthritis and flatulence.

Another member of the genus that has use as a probiotic is B. subtilis. Among the benefits of this organism as a probiotic are the relief of intestinal inflammations and urogenital infections and stopping diarrhea.

The spores of this bacterium, on the other hand, act against oxidative stress caused by keratinocyitis..

Biological controller

Bacteria of the genus Bacillus they produce a variety of substances with antibiotic properties that inhibit the growth of phytopathogenic organisms, such as non-ribosomal cyclic lipopeptides and δ-endotoxins. It is also used in the pharmacological industry to obtain antibiotics against human pathogens..

Approximately 75% of the biopesticides that are commercialized worldwide are produced based on Bacillus thuringiensis. Other species, such as B. subtilis, B. pumilus Y B. amyloliquefaciens They are also used commercially, mainly for the manufacture of fungicides.

Other uses

The industry also uses bacteria of the genus Bacillus for the commercial production of enzymes and for the production of detergents. Additionally, they are among the most widely used microorganisms in microbiology, molecular biology, or genetic engineering studies, among others..

Life cycle of species of the genus Bacillus

Under adequate conditions of nutrition, temperature, pH, atmospheric composition, among others, the cells of Bacillus grow and divide by binary fission, a form of asexual reproduction that involves DNA duplication, followed by division of the cytoplasm by a dividing septum that runs through the center of the cell, giving rise to two daughter cells.

Adverse conditions

However, when environmental conditions are adverse, vegetative cells generate endospores, which are cellular structures that do not contain ATP and have extremely latent metabolisms, which confers resistance..

Endospores form at the end of the exponential growth phase. Many other factors are known to affect the formation of endospores, such as growth temperature, environmental pH, aeration, the presence of certain minerals and sources of carbon, nitrogen and phosphorus and their concentrations. Another influence is population density.

Steps dormant spores-vegetative cells

The conversion of dormant spores to vegetative cells involves three steps: activation, germination, and outgrowth. Inactivity is broken by favorable changes in temperature or by aging of cells.

However, many species do not require such activation. When leaving dormancy, if the spore meets appropriate environmental conditions, germination is triggered, through the loss of refractance, rapid hydrolyzing of the cortex and the degradation of small acid soluble proteins that give it resistance to agents. chemical and radiation.

The protoplast of the germinated spores visibly swells due to the uptake of water, biosynthesis resumes, and a new vegetative cell emerges from the rotam spore layer, giving rise to a new period of vegetative reproduction..

Examples of species of the genus Bacillus

Bacillus cereus

Bacillus cereus

This species is a Gram positive bacterium that is commonly found in soils, water, and food in all temperate zones of the world. It is a facultative anaerobic species that can be displaced by the action of peritrichous flagella.

This bacterium is one of the main culprits of food-borne illnesses, sometimes emetic syndrome or toxic-infectious syndrome. Bacillus cereus It can also cause various non-gastrointestinal diseases, such as endocarditis, bacteremia, endophthalmitis or chronic skin infections among others..

Because it has very resistant endospores, its elimination is not achieved with cooking, nor is it eliminated by freezing, but the latter does stop its growth, so it is advisable to use temperatures below 6 ° C if the food is going to be stored. for long periods of time.

Bacillus anthracis

Bacillus anthracis

It is a species of the genus that is characterized by being immobile due to the absence of flagella, contrary to what happens with the rest of the representatives of the genus. It is Gram positive and facultative anaerobic.

This bacterium is responsible for anthrax and its pathogenicity depends on two virulence factors, a capsular polypeptide called Substance P and a protein exotoxin called Factor B.

There are numerous strains of this bacterium, whose virulence varies between them. The most virulent strains have been used as biological weapons.

Bacillus subtilis

Bacillus subtilis

Gram positive and catalase positive bacteria that inhabit the soil. It is rod-shaped with rounded edges and is 2-3 microns long by 1 micron wide. This bacterium is almost exclusively aerobic, but can survive in anoxic environments..

It is considered safe for humans, but there are isolated cases of food poisoning contaminated with this species. In these cases, the symptoms are similar to those of poisoning by Bacillus cereus.

It is one of the most studied bacterial species and researchers consider it to be the Gram positive version of Escherichia coli. It is also considered as a model organism for laboratory studies, especially in genetic manipulation studies..

Bacillus subtilis produces bactricin, a bactericidal antibiotic which is effective in treating Gram positive bacteria, such as Staphilococcus aureus. It also produces bioactive compounds with antifungal activity and substances used in the production of detergents..

Bacillus thuringiensis

Bacillus thuringiensis

Gram positive bacillus that lives in the soil, plant surfaces and in the intestines of the caterpillars of various species of butterflies and moths. It is characterized because during the sporulation process they produce protein crystals that have insecticidal properties.

Thanks to these crystals, known as δ endotoxins, the spores of Bacillus thuringensis and protein crystals have been used as biopesticides for more than 100 years.

Researchers have introduced the genetic information of this plant, through genetic engineering, to various plant species, such as potatoes, cotton or corn, so that the plants produce substances with insecticidal properties.


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