Insecticidal toxin of Bacillus thuringiensis (Bt)

B. thuring ie ns is 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.

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 ns is 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.

Target insects for Bt toxinBt 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.

Several different strains and sub species of B. thuring ie ns is exist and produce different toxins that kills specific insects.

B. thuringiensis strain Target insects

kurstaki Lepidoptera

israelensis Diptera

tenebrionis Coleoptera

aizawai Lepidoptera, Diptera

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.

.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.

B. thuring ie ns is 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.

ADVANTAGESBt 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.

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.

USE OF B. thuring ie ns is INSECTICIDAL PROTEINS IN GENETIC ENGINEERING OF PLANTSThe 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 c ry genes from B. thuring ie ns is

In 1987, the 1st report on the response of transgenic tobacco plants to the insect Manduc a s e x ta and He lio this v ire s c e 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.

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

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.

There are few reports on development of resistance in some insects to Cry proteins. The problem is being managed byCombining 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

.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.