BACILLUS: CHARACTERISTICS, REPRODUCTION, NUTRITION, DISEASES - BIOLOGY - 2025

Bacillus is a genus of bacteria in the Firmicutes division that are 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.

Bacillus flexus. Taken and edited from: Dr. Sahay.

Bacteria of this genus can be strictly 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 seabed.

The genus was used in 1835 by Christian Gottfried Ehrenberg to define rod-shaped bacteria, but was later redefined by Ferdinand Cohn and used to group spore-forming bacilli under aerobic, Gram-positive, and facultative aerobic or anaerobic conditions.

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

General characteristics

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.

The cell wall is made up of several layers of peptidoglycans intertwined, forming a strong scaffold that maintains the cell shape and contains teichoic and lipoteichoic 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.

Most of the bacteria of this genus are Gram positive, however, some are Gram variable, that is, they can stain 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 strictly aerobic, unable to survive in anoxic conditions, while other species are facultative anaerobic.

Morphology

Bacillus bacteria can be rod-shaped, straight or slightly curved, generally with a round end, even though some cells have been described as square (eg, 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 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 separate.

Thus the culture appears to be composed of individual cells and pairs of dividing cells when viewed by phase contrast microscopy. In other cases, the daughter cells can 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 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, was polyphyletic. Therefore, they carried out a rearrangement of the Bacillus sensu lato group, 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 there is the group of B. cereus. The first one 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 are they found?

Bacillus bacteria 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 dilapidated Roman wall paintings, to ultra-clean rooms in spacecraft assembly facilities.

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

Reproduction

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.

Bacillus subtilis culture. Taken and edited from: A doubt.

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, 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 that, 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 mentioned 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.

Nutrition

Most of the bacteria of the genus Bacillus are found living in the soil and their nutrition is saprophytic, 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.

Diseases

Most Bacillus species are non-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 are also common.

Anthrax or anthrax

This disease is highly contagious and is caused by the bacterium 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 start 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.

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 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 occur due to the consumption of dairy products and other foods. The causative agent is the bacterium Bacillus cereus.

This syndrome is characterized by the presence of nausea and vomiting, which appear 1 to 5 hours after the ingestion of 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

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 disease, one of the most relevant is Bacillus cereus, whose infections cause lesions with reserved prognoses.

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

Other diseases

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

Bacillus megatierium. Taken and edited from: Alexastely.

Applications

Probiotics

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 bacterium, researchers have suggested that it relieves the symptoms of irritable bowel syndrome. It 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, for their part, act against oxidative stress caused by keratinocyitis.

Biological controller

Bacteria of the genus Bacillus 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 and B. amyloliquefaciens are also used commercially, mainly for the manufacture of fungicides.

Other uses

The industry also uses bacteria of the genus Bacillus also 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.

Lifecycle

Under adequate conditions of nutrition, temperature, pH, atmospheric composition, among others, Bacillus cells grow and divide by binary fission, a form of asexual reproduction that consists of DNA duplication, followed by the division of the cytoplasm by a septum divider that crosses 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 that 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. The inactivity is broken by favorable changes in temperature or by aging of the cells.

However, many species do not require such activation. When leaving dormancy, if the spore encounters appropriate environmental conditions, germination is triggered, through the loss of refractance, a rapid hydrolyzing of the cortex and the degradation of small soluble acid 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.

Featured species

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 move through the action of peritrichous flagella.

This bacterium is one of the main culprits for food-borne illnesses, with the possibility of emetic syndrome or toxic-infectious syndrome. Bacillus cereus 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

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

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 intoxication from food contaminated with this species. In these cases, the symptoms are similar to those of Bacillus cereus poisoning.

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 Staphylococcus aureus. It also produces bioactive compounds with antifungal activity and substances used in the production of detergents.

Bacillus thuringiensis

Gram positive bacillus that inhabits 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 possess insecticidal properties.

Thanks to these crystals, known as δ endotoxins, Bacillus thuringensis spores 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 species of plants, such as potatoes, cotton or corn, so that the plants produce substances with insecticidal properties.

References

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