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New Bacterium May Aid War on Insect Pests

December 18, 1997
Caterpillar photo
The diminutive one-day-old caterpillar at left will never reach the size of the larger, seven-day-old caterpillar (right) as the result of a diet laced with a toxin derived from a bacterium with newly found insecticidal properties. The toxin inhibits feeding in addition to being toxic to a broad range of insects, making it a potentially powerful new tool in the fight against crop-destroying insects.

In the world of biological pest control, Bacillus thurengensis, Bt for short, is the king of insecticides.

For 30 years Bt, a bacterium with remarkable insecticidal properties, has been a pest-control mainstay for foresters, farmers, gardeners and homeowners in search of a safe, natural way to neutralize the bugs that bug them. As a form of biological pest control, it is the only bacterium from which widespread commercial applications have been possible, giving it, in effect, a microbial monopoly on insect control worth hundreds of millions of dollars.

But now a team of scientists from two laboratories at UW–Madison, working in collaboration with scientists from DowElanco of Indianapolis, hopes to unleash a new bacterium with similar insect-thwarting properties. The bacterium, Photorhabdus luminescens, contains a toxin that has proven effective against a broad array of insect pests – from cockroaches to boll weevils – and promises to become a potent, safe and environmentally benign weapon in the war against insect pests.

“It’s a voracious pathogen. One bacterial cell can kill an insect,” says Jerald Ensign, a UW–Madison professor of bacteriology who, with then-graduate student David Bowen, discovered and characterized the toxic potential of Photorhabdus, a widely-dispersed, multiple strain bacterium that lives inside of and in symbiosis with soil-dwelling roundworms called nematodes.

The bacteria live inside the gut of nematodes that invade insects. Once inside an insect host, the bacteria are released from the nematode, kill the insect, and set up rounds of bacterial and nematode reproduction that turns the insect into a “protein soup,” food for large numbers of nematodes.

“This makes Alien look like a cakewalk,” says Richard ffrench-Constant, a UW–Madison professor of toxicology in the department entomology.

The Photorhabdus bacteria, in fact, do Alien one better: The corpses left behind by the bacteria glow in the dark as the microbe produces luminescent proteins in addition to potent insecticides.

The findings of the Wisconsin group were reported here today (Dec. 17) at the annual meeting of the Entomological Society of America.

After establishing that Photorhabdus luminescens was indeed an effective killer of a wide variety of insects, Bowen moved as a post-doctoral fellow to the lab of ffrench-Constant where he continued work on the biochemistry of the toxin and orchestrated a nationwide survey for new toxic strains of the bacterium. So far, that survey has yielded scores of new Photorhabdus strains.

The discovery of a diverse new family of insect-killing bacteria has added importance since, in recent years, some insects have already begun to exhibit resistance to the Bt toxin, raising fears that the biological pesticide may be losing its potency. By adding an entire family of lethal bacteria to the biological pest-control arsenal, the Wisconsin and DowElanco scientists have opened a potentially broad new front in the war on insect pests since each of the Photorhabdus strains produces its own variation on the toxin.

“What we have is a natural source, almost an infinite variety” of toxic molecules, says ffrench-Constant. “We can’t afford to hook ourselves to a single bacterium or a single toxin.”

The Photorhabdus toxin and the genes that produce it have been patented jointly by the Wisconsin and DowElanco scientists through the Wisconsin Alumni Research Foundation (WARF). The technology has been licensed to DowElanco.

In concentrated doses, the toxin can be used as a spray or fed directly to insects. The greatest potential application, however, lies in transferring the toxin-producing genes from the bacteria to crop plants. Already, scientists have transferred the genes that code for Bt’s insect-thwarting properties to important crop plants. Next year, an estimated 3 million to 5 million acres of Bt transgenic corn will be planted in the Midwest alone.

“This deployment of Bt transgenic crops is perhaps the biggest artificial experiment on natural selection in insect populations since the introduction of synthetic insecticides half a century ago,” according to ffrench-Constant.

The incentive to confer crop plants with their own insecticides is huge. Farmers now spend more than $575 million annually on chemical pesticides to protect just one crop: corn.

Bowen, working with colleagues Thomas Rocheleau and Michael Blackburn in ffrench-Constant’s lab, has identified, cloned and sequenced the genes responsible for the Photorhabdus’ toxin. Clones were independently derived at DowElanco as well.

The next step, already well underway, is to move those genes to any amenable crop plant. Bringing a product to the field, however, may still take anywhere from three to five years, says ffrench-Constant.

“The need for Bt replacements is critical before we have many crops in North America expressing a limited range of Bt toxins,” says ffrench-Constant. “If we don’t have them, it’s an open invitation for natural selection to confer resistance on insects and we’ll lose that control.”

Tags: research