What is Bt?
Bt stands for Bacillus thuringiensis, which is a spore forming soil bacterium that produces protein crystals**
that are toxic to many types of insects. Bt can be found about almost anywhere. It is distributed in the soil sparsely but frequently, and that is why it can be found
almost anywhere. Bt has been found in all type of environments from beaches to
the desert to tundra type habitats. There are also over a thousand types of Bt
that produce over 200 types of protein crystals which are toxic against a wide variety of insects and some other invertebrates. Bt belongs to the bacteria family, Bacillus cerus, which cause food-poisoning in humans. Bt does not cause food poising to humans because it contains a plasmid that produces the certain protein
crystals that are toxic to insects.
** Proteins crystals
bind specifically to certain receptors in the insect’s intestine. Not all
insects have these certain receptors, which allow for high species specificity. Humans
and other vertebrates also do not have these receptors and therefore the toxin does not affect us.
History of Bt
Bt was first discovered in 1901 by a Japanese biologist, Shigetane Ishiwatari. He was investigating the cause of
the sotto disease (sudden collapse disease) that was killing large populations of silkworms.
Bt was then rediscovered in 1911 by Ernst Berliner when he had isolated a bacteria that had killed a Mediterranean
flour moth. Berliner had mentioned the existence of protein crystals but the
activity of the crystals was not discovered until much later. By 1920 farmers
were using Bt as a pesticide to kill moth larvae, since that was the only strain of Bt that was known at the time. But in 1956 Fitz-James Hannay and Angus Hannay had discovered that the reason Bt killed the moths was due
to the protein crystals. Research had begun on Bt and the Bt crystals. So by 1977 there were 13 different strains of Bt, all still only effective against moths. But also in 1977 the first strain was found that was toxic to flies.
The next strain was found in 1983 that was toxic to beetles. Today there
are thousands of strains and many encode for crystals that are toxic to a wide variety of insects. Also because of Bt’s ability to be effective and not harm the environment the government and private
industries have funded research on Bt.
How does Bt work?
The only way that Bt can be poisonous is that if it is eaten. The toxin
becomes active when it is dissolved in the high pH insect gut. These toxins then
attack the gut cells of the insect by creating holes in the lining. Bt spores
and bacteria then spill out into the gut which cause the insect to stop eating and die in a couple of days. Even though the toxin does not kill the insect immediately, parts of the plants that have been treated
with Bt will not be affected because the insect stops eating within hours. Note
that Bt does not spread to other insects and it does not cause disease outbreaks on its own.
The Bt toxin is very specific though because of the many different strains of Bt.
Each Bt strain is specific to different receptors inside the insect gut. So
the toxicity of the Bt depends on the receptors involved, and damage to the gut upon binding of the toxins to the receptors. Each species of insects has certain receptors inside their gut that will match only
certain toxins, much like a lock and a key.
|How Bt toxins work
|Click on image to enlarge
to the picture above:
and legend took from: http://www.bt.ucsd.edu/how_bt_work.html)
Insect eats the Bt toxin (crystals and spores)
The toxin binds to certain receptors in the gut and the insect stops eating
3. The crystals cause the gut wall to break down
which allow the spores and normal gut bacteria to enter body
The insect dies as spores and gut bacteria proliferate into body.