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Ghoulish Glow: Microbes Inside Worms Kill Pests
  
By Edward R. Winstead

This picture shows the bioluminescence of Photorhabdus luminescens. It was taken with film 72 hours after the bacteria infected Galleria mellonella (waxworms).

Plant growers trying to reduce their dependence on chemical pesticides are increasingly using microscopic worms as a natural means of controlling insects. The worms have bacteria in their guts, and once a worm burrows into an insect larva, the bacteria release toxins that poison the insect but don't harm the worm.

Sold in organic gardening stores and available over the Internet, the worms have been used for many years and are presumed to be harmless to humans. They're most effective in certain environments, such as greenhouses and cranberry bogs in Wisconsin. Golf courses use worms because they go underground and eliminate insects that attack the roots of grasses.

Scientists have now sequenced the genome of the killer bacterium that lives inside the worm, a microbe found all over the world called Photorhabdus luminescens. It glows in the dark and lights up the cadaver of an insect victim.

The genome project uncovered many previously unknown genes for toxins that could potentially be inserted into plants to protect them against insect pests. Tests showed that some toxins are fatal to three mosquito species, including the main African malaria mosquito.

Funded by the French Ministry of Industry, the project was a team effort involving Institut Pasteur-CNRS, INRA-Université Montpellier and Bayer CropScience in Evry, France.

This is a still from a movie showing a worm regurgitating Photorhabdus luminescens. The worm and the microbes are immersed in a tobacco hornworm.
Click here to watch the movie clip.

The goal of the Bayer CropScience team was to discover genes and proteins that might be used in commercial products. But everyone involved in the work is fascinated by Photorhabdus luminescens and motivated by intellectual curiosity. It's the only microbe known to be both a symbiont and a pathogen.

“The biology of this organism is incredible,” says Richard De Rose of Bayer CropScience. “Here you have a bacterium that kills insects but for some reason does not harm its worm host.”

The worm (Heterorhabditis bacteriophora) seems utterly dependent on the microbe. Inside the insect cadavers, the bacteria produce antibiotics and fungicides that keep other microbes from invading the nutrient-rich environment while the worm reproduces. These molecules may have potential applications in agriculture and medicine.

With the genome in hand, the next challenge is to identify genes involved in symbiosis that could be used to increase the production of the worms for the biological control of insects, says Noël Boemare of Université Montpellier, who studies insect-host interactions and has done pioneering work on Photorhabdus luminescens.

The bacterium's genome has a large amount extra DNA, including multiple copies of many genes. With 5.6 million units of DNA and some 4,800 genes, Photorhabdus luminescens has the largest genome of its class, which includes the stomach bug E. coli and the plague bacterium.

The bacterium came from a worm isolated on Trinidad and Tobago by Hervé Mauléon, and the study's findings have been published in Nature Biotechnology.

The same journal recently published a separate study in which researchers at Dow AgroSciences in Indianapolis, Indiana, took a gene from Photorhabdus luminescens and put it into a plant used in laboratory research, Arabidopsis thaliana. The plant produced a toxin that protected it against several insect pests, including the tobacco hornworm.

Whether the Photorhabdus luminescens toxin will prove effective in actual plants in nature remains to be seen, but it represents an alternative to “Bt” technology in which a toxin gene from another microbe (Bacillus thuringiensis) has been inserted into corn and cotton.

Some researchers fear that the widespread use of Bt technology could eventually lead to resistance among insects. The genome of Photorhabdus luminescens with its many insect toxins now provides some options.

“Sometimes its good to have toxins from two organisms, because if you get resistance with one you can try the other,” says Frank Kunst, who led the genome project at the Institut Pasteur in Paris.

But we are really at the beginning of this process, and we need to study all the toxins that seem interesting,” Kunst says. “A genome project is a starting point, not an end point.”

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Duchaud, E. et al. The genome sequence of the entomopathogenic bacterium Photorhabdus luminescens. Nature Biotechnology 21 , 1307-1313 (November 2003).

 

Liu, D. et al. Insect resistance conferred by 283-kDa Photorhabdus luminescens protein TcdA in Arabidopsis thaliana. Nature Biotechnology 21, 1222-1228 (October 2003).

 

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