Better Bacterial Bug Control?

Bacillus thurengensis, a potent bacterium with insecticidal properties, has been available to farmers and gardeners for three decades. It has gained wide popularity as an environmentally safe alternative to chemical pesticides. However, agricultural scientists are growing concerned that some pests may be developing resistance to BT.

The diminutive one-day-old caterpillar at left will never reach the size of the larger, seven-day-old caterpillar as the result of a diet laced with Photorhabdus luminescens

A team of scientists at the University of Wisconsin-Madison, believe Photorhabdus luminescens will ne a welcome addition to the biocontrol armamentarium. The bacterium contains a toxin that has proven effective against a wide range of insect pests - from cockroaches to boll weevils.

"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.

Like the Greeks inside the Trojan Horse, 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, he said.

The first thing the researchers did was to confirm that Photorhabdus luminescens was indeed an effective killer of a wide variety of insects. Next they determined the biochemistry of the toxin. Finally, they orchestrated a nationwide survey for new toxic strains of the bacterium. That survey has yielded scores of new Photorhabdus strains, each of which produces its own variation on the toxin. 

This diversity of the Photorhabdus luminescens is a particularly welcome discovery. Recent studies indicate that some insects have already begun to exhibit resistance to the BT toxin, raising fears that the biological pesticide may be losing its potency. Having a new legion of  lethal bacteria will allow biological pest-control specialists to open a whole new front in the war on insect pests since

"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 luminescens toxin can be used as a spray or fed directly to insects. The next step might be to  transfer 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 researchers have now identified, cloned and sequenced the genes responsible for the Photorhabdus' toxin. The next step, already well underway, is to transfer those genes to an 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."

The findings of the Wisconsin group were reported at the annual meeting of the Entomological Society of America.

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