Bacillus thuringiensis (Bt) is a soil bacteria that produces crystalline proteins called Bt toxins that are toxic to certain pest insects but safe for humans and the environment. When ingested by a susceptible insect, the Bt toxin is activated in the alkaline gut environment and inserts into the gut cells, forming ion channels that disrupt cellular metabolism and kill the insect within 24-48 hours. Bt has been used as an effective and targeted biopesticide to control major crop-damaging insects like caterpillars, beetles, and aphids, helping reduce an estimated 15% of global crop losses each year. The Bt genes that produce these crystalline toxins have been identified and classified into
2. Introduction
• Biopesticide are toxic substances produced by
living organisms that can specifically kill a
particular pest species.
• Some characteristics of pesticide :
1. Narrow target range
2. Specific mode of action
3. Slow acting & short residue effect
4. Safer to human & environmental than chemical
pesticide.
3. Continued…….
• It is estimated that about 15% of world crop
yield is lost due to pest or insect attack.
• The majority of insect that damage crop
belong to the following order.
1. Lipidoptera (ballworm)
2. Coleoptera (bettles)
3. Orthoptera (grasshoppers)
4. Homoptera (aphids)
4. Bacillus thuringinesis
• It was first discovered by Ishiwaki in 1901,but
its commercial importance was ignored until
1995.
• B.thuriginesis is a gram negative soil bacteria.
• This bacteria produces a parasporal crystalline
proteinous toxin with insecticidal activity.
• The protein produced by B.thuringinesis is
reffered as insecticidal crystalline
protein(ICP).
5. Insecticidal crystalline protein
• ICP are endotoxin protein produced by
sporolating bacteria & were originally
classified as delta endotoxin.
• The commercial mixture usually contain
spores, crystal protein & inert carriers.
• it may be mixed with chemical insecticides or
fungicides.
• The crystal protein activity usually disappears
with in 24-48 hours after application.
6. Bt toxin genes
• Several strain of B.thunginesis producing a
wide range of crystal (cry) proteins have been
identified.
• Cry gene is responsible for crystal protein.
• The cry genes are classified into a large no. of
distinct families (about 40), based on their size
& sequence similarities.
• The molecular weight of cry proteins may be
either large (130 kDa) or small (70kDa).
7. Mode of action of Cry protein
• When this parasporal crystal is ingested by the
target insect, the protoxin gets activated with
in its gut by a combination of alkaline pH (7.5
to 8.5) & proteolytic enzymes.
• This result in conversion of protoxin into a
active toxin .
• The active form of toxin protein gets itself
inserted into the membrane of the gut
epithelial cells of the insect.
8. Continued………
• This result in formation of ions channels through
which there occurs an excessive loss of cellular
ATP.
• As a consequence , cellular metabolism ceases,
insect stop feeding & becomes dehydrated &
finally dies.
• The Bt toxin is not toxic to human & animals
since the conversion of protoxin to toxin require
alkaline pH & specific proteases.
• These are absent in human & animals.
