mechanism of action of Bt toxin

1. Insecticidal toxin of
Bacillus thuringiensis (Bt)

2. B. thuring ie nsis is a gram positive, soil dwelling, spore
forming, rod shaped bacteria.
It produces a toxin or crystal protein (Bt toxin or Cry
protein) that kills certain insects.
Because of this property, Bt has been developed for insect
control. At present, Bt is the only “microbial insecticide” in
widespread use.

3. It was first discovered by in Japan by Shigetane Ishiwata in
1901, as the cause of flacherie disease in silkworms, where
it became known as Sotto disease.
B. thuring ie nsis has been used since World War I
particularly in Europe, to control insect pests.
In 1920, it was used as an alternative to DDT and
organophosphates.
Bt toxin is used as specific insecticides under trade names
as Dipel and Thuricide.

4. Target insects for Bt toxin
Bt toxins have specific activities against insect species of the orders Lepidoptera (moths and
butterflies), Diptera (flies and mosquitoes), Coleoptera (beetles), Hymenoptera (wasps, bees,
ants and sawflies) and nematodes.

5. Several different strains and sub
species of B. thuring ie nsis exist and
produce different toxins that kills
specific insects.
B. thuringiensis strain Target insects
kurstaki Lepidoptera
israelensis Diptera
tenebrionis Coleoptera
aizawai Lepidoptera, Diptera

6. How does it work?
The bacteria produces a toxin or crystal protein
(Bt toxin or Cry protein) that kills the insect larva. The
toxicity and virulence lies in the crystals.
The Bt toxin or Cry is produced when the bacteria
sporulates and is present in the parasporal crystal.
The Cry protein is made as an inactive protoxin.

7. .Conversion of the protoxin(eg 130kDa)into the active
toxin (eg 68kDa) requires the combination of a
slightly alkaline pH(7.5-8) and the action of a specific
protease found in the larva’s gut.
.The active toxin (δ endotoxins) binds to protein
receptors on the larva’s gut epithelial cell membrene.
.The toxin insert into the membrane and form cation
conducting pores making the cells permeable to ions
and protons. The influx of water that accompanies
the entrance of ions into the intestinal cells cause
them to swell and lyse.

9. B. thuring ie nsis bacteria
germinating from the spores enter
the larval haemolymph through
the damaged gut epithelium and
multiply.
The resulting bacteremia
promotes an intoxication process
leading to death of the larvae.

10. ADVANTAGES
Bt insecticides do not have a broad spectrum of
activity, so they do not kill beneficial insects.
It is nontoxic to people, pets and wildlife.
The high margin of safety recommends its use on
food crops or in other sensitive sites where
pesticides use can cause adverse effects.

11. DISADVANTAGES
Bt is susceptible to degradation by sunlight.
The highly specific activity of Bt insecticides might
limit their use on crops where problems with
several pests occur, including non-susceptible
insect.
As a stomach poison insecticide, Bt must be
eaten to be effective.

12. USE OF B. thuring ie nsis INSECTICIDAL PROTEINS IN
GENETIC ENGINEERING OF PLANTS
The Belgian company Plant Genetic Systems (now
part of Bayer CropScience) was the first company (in
1985) to develop genetically engineered (tobacco)
plants with insect tolerance by expressing cry genes
from B. thuring ie nsis
In 1987, the 1st
report on the response of transgenic
tobacco plants to the insect Manduca se xta and
He lio this vire sce ns was published.
Since 1996 transgenic crop plants (primarily
soybeens, corn and cotton) have been widely
adopted.
In 2002, more than 35 million acres of these
transgenic cotton and corn crops were planted
worldwide.

13. Bt toxins present in peanut leaves (bottom image)
protect it from extensive damage caused by Lesser
Cornstalk Borer larvae (top image)

14. CURRENT RESEARCH
 Current fortune has enabled humans to thrive
without many Bt-resistant pests. Eventually,
however, pests will evolve and acquire resistance to
Bt. Researchers at the University of California, San
Diego are studying how nematodes defend
themselves from bacterial attack. Dr. Raffi Aroian
and his colleagues attempt to analyze how Cry
proteins interact (host-toxin interaction) with gut cells
and prevent cell death. Aroian is looking at host
responses to the toxin on a genomic scale and look
to discover the transcription factors that enable
resistance in pests.

15. There are few reports on development of
resistance in some insects to Cry proteins. The
problem is being managed by
Combining or altering two or more kinds of
these proteins. For example- combination of
Bt toxins with proteins like chitinase and
Cyt1A.
Reducing the selection pressure on insects by
limiting Cry gene expression to only
economically important plants.
Production of hybrid Bt toxins

16. .It has been theorized that no known adverse environmental
effects arise when crops have been spread with Bt. 40% of
the corn crop grown in Quebec, Cananda, uses Bt.
Researchers from The Environmental Management of
Biotechnology for Regulation and Research and Agriculture
and Agri-Food Canada want to make fully certain whether
the Bt endotoxin and its gene are present in the aquatic
environment and if so, in what concentrations. Researchers
will also analyze the effects of Bt on the aquatic organisms
of the St. Lawrence River.