Bacillus thuringiensis characteristics, morphology, life cycle

1769
Alexander Pearson

Bacillus thuringiensis it is a bacterium that belongs to a wide group of gram-positive bacteria, some pathogenic and others totally harmless. It is one of the bacteria that has been studied the most due to its usefulness in agriculture..

This usefulness lies in the fact that this bacterium has the peculiarity of producing crystals during its sporulation phase that contain proteins that turn out to be toxic to certain insects that constitute true pests for crops..

Crystals of B. thuringiensis toxin. By Jim Buckman is credited and the original uploader is P.R. Johnston. (w: en: Image: Bacillus thuringiensis.JPG) [Public domain], via Wikimedia Commons

Among the most outstanding characteristics of the Bacillus thuringiensis are its high specificity, safety for man, plants and animals, as well as its minimal residuality. These attributes allowed it to position itself as one of the best options for the treatment and control of pests that plagued crops..

The successful use of this bacterium became evident in 1938 when the first pesticide manufactured with its spores emerged. From then on the history has been long and through it the Bacillus thuringiensis as one of the best options when it comes to controlling agricultural pests.

Article index

  • 1 Taxonomy
  • 2 Morphology
  • 3 General characteristics
  • 4 Life cycle
    • 4.1 The toxin
  • 5 Uses in pest control
    • 5.1 Mechanism of action of the toxin
    • 5.2 Bacillus thuringiensis and pesticides
    • 5.3 Bacillus thuringiensis and transgenic foods
  • 6 Effects on the insect
  • 7 References

Taxonomy

The taxonomic classification of the Bacillus thuringiensis it is:

Domain: Bacterium

Edge: Firmicutes

Class: Bacilli

Order: Bacillales

Family: Bacillaceae

Gender: Bacillus

Species: Bacillus thuringiensis

Morphology

They are rod-shaped bacteria with rounded ends. They present a pertric flagellation pattern, with flagella distributed over the entire cell surface.

It has dimensions of 3-5 microns long by 1-1.2 microns wide. In their experimental cultures, circular colonies are observed, with a diameter of 3-8 mm, with regular edges and a “ground glass” appearance..

When observed with the electron microscope, the typical elongated cells are observed, united in short chains.

This species of bacteria produces spores that have a characteristic ellipsoidal shape and are located in the central part of the cell, without causing deformation of the cell..

General characteristics

First of all, the Bacillus thuringiensis It is a gram-positive bacterium, which means that when subjected to the Gram staining process it acquires a violet coloration.

Likewise, it is a bacterium characterized by its ability to colonize various environments. It has been possible to isolate it on all types of soils. It has a wide geographical distribution, having been found even in Antarctica, one of the most hostile environments on the planet..

It has an active metabolism, being able to ferment carbohydrates such as glucose, fructose, ribose, maltose and trehalose. It can also hydrolyze starch, gelatin, glycogen and N-acetyl-glucosamine.

In the same vein, the Bacillus thuringiensis it is catalase positive, being able to decompose hydrogen peroxide into water and oxygen.

When it has been cultured on blood agar medium, a pattern of beta hemolysis has been observed, which means that this bacterium is capable of totally destroying erythrocytes..

Regarding its environmental requirements for growth, it requires temperature ranges from 10 - 15 ° C to 40 -45 ° C. Similarly, its optimal pH is between 5.7 and 7.

The Bacillus thuringiensis it is a strict aerobic bacteria. It must be in an environment with ample oxygen availability.

The distinctive feature of the Bacillus thuringiensis is that during the sporulation process, it generates crystals made up of a protein known as delta toxin. Within these two groups have been identified: the Cry and the Cyt.

This toxin is capable of causing the death of certain insects that constitute true pests for various types of crops.

Lifecycle

B. thuringiensis It has a life cycle with two phases: one of them characterized by vegetative growth, the other by sporulation. The first of them occurs in favorable conditions for development, such as nutrient-rich environments, the second in unfavorable conditions, with a shortage of food substrate.

The larvae of insects such as butterflies, beetles or flies, among others, when feeding on the leaves, fruits or other parts of the plant, can ingest endospores of the bacteria B. thuringiensis.

In the digestive tract of the insect, due to its alkaline characteristics, the crystallized protein of the bacterium is dissolved and activated. The protein binds to a receptor on the insect's intestinal cells, forming a pore that affects electrolyte balance, causing the insect's death..

Thus, the bacterium uses the tissues of the dead insect for its feeding, multiplication and formation of new spores that will infect new hosts..

Toxin

The toxins produced by B. thuringiensis they present highly specific action in invertebrates and are harmless in vertebrates. Parasporal inclusions of B. thuringensis possess diverse proteins with diverse and synergistic activity.

B. thuringienisis It has various virulence factors that include, in addition to Cry and Cyt delta endotoxins, certain alpha and beta exotoxins, chitinases, enterotoxins, phospholipases and hemolysins, which enhance its efficiency as entomopathogens.

The toxic protein crystals of B. thuringiensis, are degraded in the soil by microbial action and can be denatured by incidence of solar radiation.

Uses in pest control

The entomopathogenic potential of Bacillus thuringiensis has been highly exploited for more than 50 years in the protection of crops.

Thanks to the development of biotechnology and advances in it, it has been possible to use this toxic effect through two main routes: production of pesticides that are used directly on crops and the creation of transgenic foods..

Mechanism of action of the toxin

In order to understand the importance of this bacterium in pest control, it is important to know how the toxin attacks in the insect's body..

Its mechanism of action is divided into four stages:

Cry protoxin solubilization and processing: crystals ingested by the insect larvae dissolve in the intestine. Due to the action of the proteases present, they are transformed into active toxins. These toxins cross the so-called peritrophic membrane (protective membrane of the cells of the intestinal epithelium).

Binding to receivers: toxins bind to specific sites that are located in the microvilli of the intestinal cells of the insect.

Insertion into the membrane and pore formation: Cry proteins insert into the membrane and cause total tissue destruction through the formation of ion channels.

Cytolysis: death of intestinal cells. This occurs through several mechanisms, the best known being osmotic cytolysis and the inactivation of the system that maintains the pH balance..

Bacillus thuringiensis and pesticides

Once the toxic effect of the proteins produced by the bacteria was verified, their potential use in the control of pests in crops was studied..

Many studies have been carried out to determine the pesticidal properties of the toxin produced by these bacteria. Due to the positive results of these investigations the Bacillus thuringiensis it has become the most widely used biological insecticide worldwide to control pests that damage and negatively affect various crops.

Source: Pixabay.com

Bioinsecticides based on Bacillus thuringiensis they have evolved over time. From the first ones that only contained spores and crystals, to the so-called third generation ones that contain recombinant bacteria that generate the bt toxin and that have advantages such as reaching plant tissues.

The importance of the toxin produced by this bacterium is that it is not only effective against insects, but also against other organisms such as nematodes, protozoa and trematodes..

It is important to clarify that this toxin is totally harmless in other types of living beings such as vertebrates, a group to which humans belong. This is so because the internal conditions of the digestive system are not ideal for its proliferation and effect..

Bacillus thuringiensis and transgenic foods

Thanks to technological advances, especially the development of recombinant DNA technology, it has been possible to create plants that are genetically immune to the effect of insects that wreak havoc on crops. These plants are known generically as transgenic foods or genetically modified organisms..

This technology consists of identifying within the bacterium's genome the sequence of genes that encode the expression of toxic proteins. Later, these genes are transferred to the genome of the plant to be treated..

When the plant grows and develops, it begins to synthesize the toxin that was previously produced by the Bacillus thuringiensis, being then immune to the action of insects.

There are several plants in which this technology has been applied. These include corn, cotton, potatoes, and soybeans. These crops are known as BT corn, BT cotton, etc..

Of course, these transgenic foods have generated some concern in the population. However, in a report published by the United States Environment Agency it was determined that these foods, to date, have not manifested any type of toxicity or damage, neither in humans nor in higher animals..

Effects on the insect

The crystals of B. thuringiensis they dissolve in the intestine of the insect with high pH and protoxins are released, and other enzymes and proteins. Thus, protoxins become active toxins that bind to specialized receptor molecules on the cells of the intestine..

Toxin by B. thuringiensis produces in the insect cessation of ingestion, intestinal paralysis, vomiting, imbalances in excretion, osmotic decompensation, general paralysis and finally death.

Due to the action of the toxin, serious damage occurs in the intestinal tissue that prevents its functioning, affecting the assimilation of nutrients.

"Caenorhabditis elegans" intestine infected with "Bacillus thuringiensis'. Source: www.researchgate.net

It has been considered that the death of the insect could be caused by the germination of spores and the proliferation of vegetative cells in the hemocele of the insect.

However, it is thought that mortality would depend more on the action of commensal bacteria that inhabit the intestine of the insect and that after the action of the toxin of B. thuringiensis would be capable of causing septicemia.

Toxin from B. thuringiensis It does not affect vertebrates, because the digestion of food in the latter is carried out in acidic environments, where the toxin is not activated.

Its high specificity in insects stands out, especially known for Lepidoptera. It is considered harmless for most of the entomofauna and has no harmful action on plants, that is, it is not phytotoxic.

References

  1. Hoffe, H. and Whiteley, H. (1989, June). Insecticidal Crystal Proteins of Bacillus thuringiensis. Microbiological Review. 53 (2). 242-255.
  2. Martin, P. and Travers, R. (1989, October). Worldwide Abundance and Distribution of Bacillus thuringiensis Applied and Environmental Microbiology. 55 (10). 2437-2442.
  3. Roh, J., Jae, Y., Ming, S., Byung, R. and Yeon, H. (2007). Bacillus thuringiensis as a Specific, Safe and Effective Tool for Insect Pest Control. Journal of Microbiology and Biotechnology. 17 (4). 547-559
  4. Sauka, D. and Benitende G. (2008). Bacillus thuringiensis: generalities. An approach to its use in the biocontrol of lepidopteran insects that are agricultural pests. Argentine Journal of Microbiology. 40. 124-140
  5. Schnepf, E., Crickmore, N., Van Rie, J., Lereclus, D., Baum, J., Feitelson, J., Zeigler, D., and Dean H. (1998, September). Bacillus thuringiensis and Its Pesticidal Crystal Protein. Microbiology and Molecular Biology Reviews. 62 (3). 775-806.
  6. Villa, E., Parrá, F., Cira, L. and Villalobos, S. (2018, January). The genus Bacillus as biological control agents and their implications for agricultural biosecurity. Mexican Journal of Phytopathology. Online posting.

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