Bacillus thuringiensis Cry proteins and the bacterium itself are interchangeable when used as pesticides. If the plant has been genetically modified, it contains the Cry protein within itself. If
a pesticide has been used, the plant is coated in the bacteria. Both accomplish much of the same effect, but the Genetically Modified method tends to work better.
It is best to divide the process into three sections.
In Part A, the active crystal protein is attached to the Bt spore. This spore is ingested by an unfortunate organism, and the work begins. The spore and crystal enter the digestive tract, where they make their way to the
In Part B The crystal surrounding the protein breaks off from the spore and begins to dissolve. Once the crystal has been completely dissolved, the toxin is activated. This entire step occurs in the gut of the insect, and is the first specificity test for Bt. In order for the crystal to be dissolved and the toxin to become activated, the pH level must be within a very limited range. If the pH is too low (acidic), the acid will dissolve the toxin before it can take affect, if the pH is too high (basic), there will not be enough acidity to dissolve the crystal, and the toxin will pass through the organism's digestive tract without being activated. Thus, only specific organisms will have the correct pH balance to release the toxin.
In Part C the toxin actually takes affect. The toxin binds itself to receptors in the gut. In this illustration, it shows small square shapes along the gut membrane, which are (in actuality) folds along the gut membrane. Once the toxin binds itself to the membrane, it begins to bore holes through it. It completely perforates the gut membrane. Multiple toxins will bind and follow these steps, until the creature has been paralyzed, or starves to death. The creature is paralyzed because its stomach acids leak out into the rest of the body causing serious damage (pleasant). It is for this reason that the insect is unable to digest, and can therefore starve to death with a full stomach. This step is another of the specificity tests of Bt. If the folds in the gut membrane are too large or too small the toxin will not be able to bind to the gut membrane. Only specific organisms will have the correctly sized folds, and thus, will be affected.
Bacillus thuringiensis gives a caterpillar the one-two
Courtesy: International Programme on Chemical Safety