Bt Toxins, Used By Organic Farmers As An Environmentally Safe Insecticide, Found To Kill Parasitic Roundworms

February 19, 2003
By: Kim McDonald
Photo Credit: J. Wei and K. Hale, UCSD

Biologists at the University of California, San Diego have discovered that Bacillus thuringiensis, or Bt — a bacterium that produces natural protein insecticides that have been used by organic farmers for five decades — can also produce similar natural proteins that kill nematodes.

Photos of rat parasitic nematode, N. brasiliensis, before and after application of Bt toxin Cry21A. (at right, sick, small worms)

The discovery could pave the way for the development of an inexpensive and environmentally safe means of controlling the parasitic roundworms that each year destroy billions of dollars in crops, cause debilitating diseases in farm animals and pets, and now infect a quarter of the world's human population. The scientists' findings appear in the March 4 issue of the Proceedings of the National Academy of Sciences, which is making their paper available this week in its early online edition.

Major parasitic roundworm diseases in humans include ascariasis, which affects 1.5 billion people worldwide; hookworm, which infects 1.3 billion people; and elephantiasis, which affects 120 million people. Other parasitic nematodes are major agricultural pests, affecting such economically important crops as corn, soybeans, potatoes and tomatoes. They are also a problem in horses, livestock and pets.

Scientists have been increasingly concerned about parasitic nematodes developing resistance to the drugs now being used to treat or prevent their infestations. But an even larger impediment to the widespread use of those drugs is that they are costly, as any pet owner who must purchase heartworm preventative knows.

That's not the case with Bt toxins, which have played an important role in controlling insects, such as mosquitoes that carry disease, in third world countries and are now being used in genetically modified cotton, corn and other crops to control caterpillars and beetles.

"Not only are Bt toxins relatively easy to make, but they are extremely safe to humans and vertebrate animals," says Raffi V. Aroian, an assistant professor of biology at UCSD who headed the study. "All of the data show that these crystal proteins are non-toxic to animals with backbones. What our discovery suggests is the potential for preventing not only billions of dollars worth of agricultural damage from parasitic roundworms each year, but also the potential for preventing some important and debilitating forms of human and animal disease."

The discovery was made by a team of biologists working in Aroian's laboratory — Jun-zhi Wei, Kristina Hale, Cynthie Wong and Su-chiung Fang. Nematologists Lynn Carta of the USDA's Agricultural Research Service in Beltsville, Md., and Edward Platzer of UC Riverside also contributed to the study, which was supported by the National Science Foundation, Burroughs-Wellcome Foundation, the Beckman Foundation and the University of California BioSTAR program.

The UCSD biologists sought to investigate the potential for Bt toxins — which attack and dissolve the intestines of their insect hosts — as an anti-roundworm agent after determining that a specific Bt crystal protein they were studying, Cry5B, destroyed the intestines of the common laboratory nematode, C. elegans. They also found in previous experiments that two other Bt crystal proteins, Cry6A and Cry14A, had the ability to significantly reduce the ability of C. elegans to produce numerous young.

To determine whether these effects applied to other nematode species, the scientists investigated the effects of seven crystal proteins related to and including Cry5B and Cry6A, on five phylogenetically diverse free-living nematode species that feed on bacteria — C. elegans, Pristionchus pacificus, Panagrellus redivivus, Acrobeloides sp. and Distolabrellus beechi — and the free-living stage of a different species — Nippostrongylus brasiliensis — which in other stages is an intestinal parasite in rats. The researchers discovered that all six of these nematodes, which were also chosen because they are fairly diverse and can be raised easily in the laboratory, are susceptible in varying degrees to the crystal proteins — either by killing them, damaging their intestines or reducing their brood size. They also found that not all of the related crystal proteins killed nematodes, suggesting that only a limited number of the known crystal proteins target this invertebrate phylum.

The UCSD discovery represents the first time scientists have verified that Bt toxins can widely affect nematodes. "Our finding that there is a family of crystal proteins that can kill nematodes is the first in the scientific literature, although it was recognized in patents filed by the Mycogen Corporation," says Aroian. "And the most important part of the discovery is that we can kill at least one nematode that is a mammalian parasite, which suggests that these crystal proteins can be used against nematode parasites in humans."

Whether such Bt crystal proteins can negatively affect some or all of the many species of beneficial soil-dwelling nematodes that control insect pests will require further study before they can be widely used on agricultural crops. However, the Bt strains that produce these crystal proteins, which multiply in the intestines of nematodes, suggest to the scientists that soil-dwelling nematodes may have contributed to the evolution and spread of the Bt bacterium.

"It seems plausible that a soil bacterium might take advantage of the fact that soil nematodes use bacteria as a food source and evolve crystal proteins to help it propagate inside the host nematode once its spores and crystals are ingested," the scientists write.

"The most important conclusion of these studies is that Bt crystal proteins have potential in controlling nematode pests that infect animals and plants," the authors add in their paper. "In particular, the effectiveness of Cry5B, Cry14A and Cry21A against the free-living stage of an animal parasite, N. brasiliensis, is promising. Although it remains to be seen whether these toxins are effective against other parasites or against nematodes in parasitic stages, these results demonstrate that a parasitic nematode can express the proper molecular elements for responding to Bt toxins. Given the very low toxicity of Bt crystal proteins in general toward vertebrates, Bt crystal proteins may one day provide safe, cost-effective control of nematode parasites, such as those that infect over a quarter of the human population."