Dr. John J. Brown, Chair, WSU Department of Entomology
A diseased silkworm was discovered by a Japanese bacteriologist (Ishawata) in 1901. The disease causing organism was identified as Bacillus thuringiensis (Bt) by a German scientist (Berliner) a decade later, and has been used to control select groups of insects for decades since. Bt was first sold under the trade name Sporeine, in 1938. The active ingredient in Bt is a toxic protein that actually binds to specific sites on the midgut cells of the insect. After several of these proteins bind, they create a cavity in the gut cell, allowing leakage and total destruction of the gut. Consequently, the pest insect stops feeding and eventually dies. Applications of Bt are quickly lost if rain soon follows the time of application.
Bt received considerable attention from biotechnology companies, and since 1980 nineteen different companies have had research programs targeting the d-endotoxin protein. The d-endotoxin protein was characterized and the genes responsible for synthesis of this toxin inserted into plants. These genetically altered plants with "built-in" insecticides include: corn (Maximizer, or Yield Guard,), cotton (NuCOTN,), potatoes (NewLeaf,), etc. A serious concern is the potential for pests to develop resistance to the d-endotoxin when it is constantly present within the plant's genome. This is essentially how resistance to DDT developed. DDT was persistent in the environment, and insects that tolerated the insecticide passed their resistance genes on to their progeny. More lethal agents in addition to the dendotoxin of Bt are needed for incorporation into the transgenic crops.
Now, a team of researchers, led by Dr. Richard ffrench-Constant at the University of Wisconsin, have reported on a new bacterial protein toxin isolated from an entomophagous nematode (Science[1998] 280:2129). Some Heterorhabditae nematodes puncture their host's body and infuse the wounded host with bacteria that essentially pre-digest the inside of the host. The nematode then ingests the liquid diet for nutrients. The bioluminescent bacterium researched by Dr. ffrench-Constant is called Photorhabdus luminescens. It produces four (A, B, C, D) high molecular weight protein complexes that were investigated by the Wisconsin research team. When tobacco hornworm larvae were fed protein complex A or D, it was as potent as the Bt d-endotoxin.
Some insect populations have developed resistance to Bt. The discovery of Photorhabdus luminescens may allow alternation or co-deployment of both the bacterial toxin carried by the nematode and the Bt toxin, thereby delaying the development of resistance to either toxin alone.
Reference: Bowen, D., T. A. Rocheleau, M. Blackburn, O. Andreev, E. Golubeva, R. Bhartia, and R.H. ffrench-Constant. (1998). Insecticidal toxins from the bacteria Photorhabdus luminescens. Science 280:2129-2132.
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