How Bacillus Thuringiensis Works in Agriculture and Crop Protection
It is also known as Bt, it is a gram-positive bacteria found in soil. It is commonly used as a pesticide. It is also found naturally in the guts of butterflies, several types of moths, and caterpillars. This paradise can also cause disease in some of the moths such as Cadra calidella. The B. thuringiensis was first discovered in 1902, in silkworms by a sericulture engineer of Japan and named it B. Sotto. Then later in 1911, Ernst Berliner a German microbiologist rediscovered the Bt in the diseased moths found in Thuringia, thus the species name. Bt is closely related to B. cereus and B. anthracis in which it is found in soil and can cause anthrax respectively. All these three organisms vary mainly depending on the plasmids present in them.
Bacteria Bacillus Thuringiensis
The scientific classification of Bt is as follows:
Domain: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Bacillaceae
Genus: Bacillus
Species: B. thuringiensis
Thus leading to the binomial nomenclature named Bacillus thuringiensis.
Structure:
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As the Bacillus thuringiensis is a gram-positive bacteria it has a thick cell wall. This cell wall is made up of peptidoglycan. In between the cell wall and a plasma membrane a periplasmic space is present. This space is a small section that is responsible for protection and biosynthesis. It has a circular-shaped chromosome. It has many plasmids. The Bt strains produce the crystal proteins during the process of sporulation called -endotoxins.
Use of Bacillus Thuringiensis
The environment protection agency approved a CRY 3A Bt toxin that was produced from potato plants in 1985. It is the first human-modified pesticide that produces crops.
The European corn borer was killed by Bt Cry protein produced from genetically modified maize in 1996. The Bt genes were introduced in the corn there and it killed the corn rootworm larvae.
By 2014 in India more than seven million farmers have adopted the Bt cotton by occupying the 2 million hectares
Bacillus Thuringiensis Products
Bt is a naturally occurring bacteria, that consists of spore where the protein inside it is toxic. This spore is persistence. Depending on the subspecies the toxic proteins differ. When the toxic protein gets inside the gut of the insect, the toxic crystal is released and it makes the gut highly alkaline. This makes the insect get favored from the digestive juices thus the insect dies due to poisoning. But in the case of mammals, birds, and fish due to their acidic gut, it makes it difficult to survive the bacteria.
Some of the Bacillus thuringiensis products are used as,
Bt Kurstaki (Bt-k) – Bacillus thuringiensis var kurstaki: Bt-k is a naturally occurring bacteria. It helps in the ideal control of tomato hornworm, cabbage looper, and other leaf-eating caterpillars found in trees, tomatoes, shrubs, and other vegetables. In the first and second instars if the caterpillars are given with the Bt-k it is more effective. It can degrade quickly in the sunlight, thus under heavy insect pressure, reapplication is required.
Bt Israelensis (Bt-i) – Bacillus thuringiensis var israelensis: It is a biological pesticide that is highly specific and used against mosquito, fungus gnut larvae, and black fly. Anyplace that has standing water Bt-i can be injected in order to kill the larvae, once it is injected it can kill up to 95% of the larvae within 24 hours.
Bt San Diego (Bt-sd) – Bacillus thuringiensis var san Diego and Bt tenebrionis (Bt-t) – Bacillus thuringiensis var tenebrionis: A Colorado potato beetle has very resistant to multiple applications of insecticides. These two can control the leaf-eating beetle species to a limited range. These can be applied only to the larvae as in adults the effect is found to be very small.
Bacillus Popilliae: Milky spore is a disease caused to Japanese beetle larvae succumb to the Bacillus popilliae bacteria. The bacteria spreads normally by killing each of the larvae.
Bacillus Thuringiensis Application
Used in biotechnology, through genetic engineering, it can be introduced into the crops. Bt crop varieties are invented to produce the protein toxic to certain specific insects and are used in the areas where the pests are targeted. Bt cotton, corn, rice, and potato commercial production was increased in many of the countries. Bacillus thuringiensis biopesticide has become a boon for the farmers to protect their crops.
Golden rice that is rich in vitamin A is produced.
Conclusion:
The process of creating a new species with the selected traits is genetic engineering. Bt is a soil-dwelling bacterium that produces certain toxic proteins. These proteins are dangerous to herbivorous insects. To apply Bacillus thuringiensis spray some tips have to be followed, if the larvae are small and Bt is injected at that time then it is more effective. If the pH is more than 8 then this level activates the toxin in the insect’s gut thus this pH level is to be managed. If the spray is mixed then it should be applied within twelve hours for effective results and the spray is to be applied to both sides of the leaves. The bacteria can live in the ground for years but if it gets absorbed then the UV radiation from the sun can destroy it. Thus most people spray the Bt during the evenings to work on the insects overnight as during the daytime the bacterium can be inactivated by the radiations. These are sensitive to temperature and can be stored within 50 to 60 F. While storing the bacillus thuringiensis insecticide some of the precautions have to be followed.
FAQs on Bacillus Thuringiensis: Key Features, Mechanism & Applications
1. What is Bacillus thuringiensis and what is its main application in agriculture?
Bacillus thuringiensis, often abbreviated as Bt, is a naturally occurring, soil-dwelling bacterium. Its primary application in agriculture is as a biological pesticide, or biopesticide. It is widely used in organic farming and through genetically modified crops to selectively control pests like lepidopteran larvae (moths, butterflies), coleopterans (beetles), and dipterans (flies, mosquitoes).
2. What specific toxin is produced by Bacillus thuringiensis, and how does it work?
Bacillus thuringiensis produces a protein crystal during a specific phase of its growth. This crystal contains a toxic insecticidal protein known as cry protein or Bt toxin. Initially, this protein is an inactive protoxin. When an insect ingests it, the alkaline pH of the insect's gut dissolves the crystal and activates the toxin. The activated toxin then binds to the surface of midgut epithelial cells, creating pores that cause cell swelling and lysis, ultimately leading to the insect's death.
3. Why is the Bt toxin generally considered safe for humans, fish, and other mammals?
The Bt toxin is safe for humans and most non-target animals because its activation requires a highly alkaline pH, which is characteristic of an insect's gut. The human stomach, being highly acidic, does not activate the protoxin. Furthermore, mammals lack the specific gut receptors to which the activated toxin must bind to exert its effect. This specificity makes it a safe and effective biopesticide.
4. Why doesn't the Bt toxin kill the Bacillus bacterium that produces it?
The Bt toxin does not harm the bacterium itself because it is produced in an inactive, crystalline form called a protoxin. This protoxin is harmless and requires the unique conditions of an insect's gut—specifically, high alkaline pH—to become activated and toxic. Since these conditions do not exist within the bacterium, it remains unaffected by its own potent protein.
5. What is the significance of the 'cry' gene in creating genetically modified crops like Bt cotton?
The gene responsible for producing the cry protein in Bacillus thuringiensis is called the 'cry' gene. In biotechnology, scientists isolate this gene and introduce it into the genome of plants, such as cotton, corn, or brinjal. This creates a genetically modified (GM) crop, like Bt cotton, that produces its own insecticidal protein. As a result, the plant becomes inherently resistant to specific pests, reducing the need for external chemical pesticide sprays.
6. How does Bt cotton offer better pest protection compared to just spraying Bt pesticides?
Bt cotton provides more consistent and effective protection for several reasons:
- Continuous Expression: The plant produces the Bt toxin in its tissues continuously as it grows, offering protection throughout the season. Sprays, on the other hand, can be washed away by rain and have a limited period of effectiveness.
- Targeted Delivery: The toxin is delivered directly to the pest when it feeds on any part of the plant (e.g., bollworms boring into the cotton boll). This is often more effective than surface-level sprays.
- Reduced Environmental Impact: It minimises the use of broad-spectrum chemical pesticides, which can harm beneficial insects and pollute the environment.
7. Are all types of insects susceptible to a single strain of Bacillus thuringiensis?
No, a single strain of Bacillus thuringiensis is not effective against all insects. The bacterium is highly specific. Different strains of Bt produce different types of cry proteins, and each protein is toxic only to a narrow group of insects. For example, the proteins that kill moth larvae are different from those that kill beetle larvae. This specificity is a major advantage, as it helps preserve beneficial insects like pollinators while targeting only the intended pests.
