Bacillus thuringiensis (Bt) is an entomopathogenic bacterium recognized for its effectiveness in producing insecticidal proteins that are lethal to several orders of insects, including Lepidoptera, beetles, and mosquitoes. This organism has become a vital tool in biological control, offering a more environmentally friendly and less harmful alternative to conventional chemical insecticides.
The increasing environmental damage associated with the excessive use of synthetic pesticides has led to renewed interest in the development and application of biological insecticides, where Bacillus thuringiensis It stands out as one of the most outstanding. Its high specificity of action and low environmental impact make it a bioinsecticide of choice, considered the most successful in the implementation of pest control programs in the field. In addition, there are methods such as ecological insecticides that complement its use.
In addition to being a selective insecticide, Bt has the ability to generate a wide variety of insecticidal proteins that attack only a narrow group of organisms, minimizing the possibility of affecting non-target species, including beneficial insects. The action of these insecticides depends on the combination of spores and crystals from different isolates of the bacteria, which are marketed in multiple formulations intended to control Lepidopteran larvae, beetles, and mosquitoes.
Insecticidal proteins from Bacillus Thuringiensis
Knowledge of the mode of action of Bacillus thuringiensis It is crucial to understand how resistance can develop in insects and to improve existing insecticidal products. Research conducted at institutions such as the University of Valencia has reviewed the mechanisms of action of the most relevant insecticidal proteins present in these products, as well as the use of ecological insecticides in orchards.
Bacillus thuringiensis During its sporulation phase, it produces several crystalline forms containing proteins known as "Cry" and "Cyt," while in its vegetative phase, proteins called Vip are generated. These proteins play a fundamental role in pest control and are remarkably specific in their action, as they primarily affect the larvae of specific insects.
Although the cytolytic toxins Cyt They have been the subject of little study and are used to control Diptera. In contrast, the insecticidal proteins Cry and Vip are the most studied due to their high specificity and efficacy in pest control.
Mode of Action of Bt Proteins
Ingestion:
The first step in the mechanism of action of Bacillus thuringiensis is the ingestion of particles by susceptible species. Unlike other insecticides that can act by contact, Bt and its proteins must be ingested to be effective.
Solubilization:
Once the Cry proteins have been ingested, the next step is their solubilizationThis process involves the breakdown of the crystalline structure and the release of protoxins. Solubilization is a key factor in the specificity of Cry proteins, since an appropriate intestinal environment, such as a specific pH, is required for this process to occur. The advantages of this type of bioinsecticides are highlighted by the use of insecticides.
On the other hand, Vip proteins do not require this step, as they are secreted in a soluble form directly from the bacteria.
Activation:
Solubilized Cry protoxins and Vip protoxins are processed by endogenous protease enzymes in the intestinal fluids of insects, producing an active toxin. This process is primarily carried out by trypsins and chymotrypsins, the most abundant enzymes in insect guts.
Research has shown that variability in susceptibility of the species may depend on the ability to activate these toxins, a crucial factor for the development of resistance in insects.
Crossing the Intestinal Membrane:
Once the toxins are activated, they must pass through the midgut membraneThis membrane is rich in chitin and acts as a physical barrier separating the food content from the epithelial cells. Although it provides physical protection, chitinase enzymes, both endogenous and exogenous from Bt, can degrade chitin, thus compromising the effectiveness of this protective mechanism.
Interaction with the Midgut
After passing the peritrophic membrane, the Cry and Vip proteins interact with the membrane of the intestinal cells, which act as the target cells for toxins. Identifying the molecules that specifically bind to these toxins has been a key challenge in Bacillus thuringiensis research over the past few decades. There are also other alternatives, such as homemade insecticides for plants, which can be used together.
Experimental evidence suggests that at least three types of membrane proteins They can function as receptors for Cry proteins in insects: aminopeptidases N (APN), cadherins, and ABC transporters. However, little is known about the potential specific receptors for Vip toxins.
Insect Death and Dispersal of Bacillus Thuringiensis
The most widely accepted model to explain the toxic activity of Bt proteins is based on their ability to form pores in membranes of target cells in the gut. The binding of toxins to their specific receptors results in the creation of a structure that perforates the cell membrane, causing a loss of integrity that leads to cell rupture and septicemia. This allows spores and other pathogenic bacteria to enter the insect's system, contributing to its eventual death.
Once the insect has died, Bacillus thuringiensis benefits from this niche, where it can proliferate and sporulate, thus ensuring its dispersal into the environment. This mode of action not only highlights the effectiveness of the bioinsecticide but also underscores the importance of Bacillus thuringiensis in biological pest control. For more effective use, it is advisable to learn more about insecticides for plants.
