Question

Crystals of Bt toxin produced by some bacteria do not kill the bacteria themselves because - (a) bacteria are resistant to the toxin (b) toxin is immature; (c) toxin is inactive: (d) bacteria encloses toxin in a special sac.

Short answer

The crystals of Bt toxin do not kill the bacteria themselves because the toxin is inactive (protoxin) within the bacteria.

Step-by-step solution

Identify the nature of the Bt toxin in bacteria

Bt toxin when produced within the bacteria Bacillus thuringiensis, is in an inactive form called protoxin. These protoxins are crystalline and do not harm the bacteria because they are not active.

Determine the reason why Bt toxin doesn't kill the bacteria itself

The reason behind the bacteria's immunity to its own toxin is related to the inactive state of the crystal. It would require specific conditions usually found in the target insect's gut to activate and pose a lethal threat.

Choose the correct answer

Based on the information about the protoxin's state within the bacteria, the correct answer is that the toxin is inactive.

Key concepts

Bacillus thuringiensis

The soil-dwelling bacterium Bacillus thuringiensis, often abbreviated as Bt, plays a remarkable role in agricultural pest control. Discovered in the early 20th century, this bacterium naturally produces a toxin that is lethal to many insect species, particularly those in the Lepidoptera order, which includes moths and butterflies.

Bacillus thuringiensis creates crystal proteins known as cry toxins during its sporulation phase. These proteins are highly specific to target insects and are considered safe for humans, animals, and beneficial insects due to this specificity. When ingested by susceptible insects, the alkaline environment of the insect's gut activates these toxins, causing their gut cells to break down, ultimately leading to death.

Farmers and gardeners often utilize Bt-based insecticides as an eco-friendly alternative to chemical pesticides, greatly reducing the negative impact on the environment. Due to its advantage of being naturally occurring and having a targeted effect on pests, it is widely employed in integrated pest management programs.

Protoxin

The concept of protoxin is central to understanding why Bacillus thuringiensis is not affected by its own toxin. A protoxin is an inactive form of a toxin that must be activated to exhibit its toxic effects. In the case of Bt, the protoxin forms crystalline inclusions within the bacterium’s spore.

These crystalline protoxins, also referred to as Cry proteins, remain inactive and harmless within the bacterial cell, posing no threat to its survival. It is only after being consumed by the target pest that the protoxins become lethal. The activation occurs in the highly alkaline environment of the insect's digestive system, where certain enzymes cut specific bonds in the protoxin molecule. This enzymatic cleavage transforms the protoxin into an active toxin, which can then bind to receptors on the gut cells of the insect, disrupting its digestive processes and leading to the insect's death.

The fact that protoxin remains inactive within the bacterial cells is an excellent display of nature's precision in designing mechanisms that protect the producer while selectively targeting specific pests.

Genetic Engineering in Pest Control

Expanding on the natural abilities of Bacillus thuringiensis, genetic engineering has provided a powerful tool in pest control. Scientists have been able to transfer genes responsible for producing Bt toxin from the bacterium into crops. This revolutionary accomplishment has led to the creation of genetically modified (GM) plants that express Bt toxin, providing them with intrinsic resistance to specific insect pests.

As a result, these GM crops can significantly decrease the need for external chemical pesticides, reducing environmental pollution and health risks associated with pesticide use. Crops like Bt cotton and Bt corn are now cultivated worldwide, helping farmers to fight against damaging pests more effectively and sustainably.

However, the use of genetically engineered crops does come with considerations such as potential effects on non-target species, resistance development in pests, and societal and ethical concerns about GM organisms. Nevertheless, the integration of Bt genes into crops remains a prominent example of how genetic engineering can contribute to sustainable agriculture and pest management strategies.