Insecticide gets help from gut bacteria.

The world's most widely used organic insecticide relies on an insect's normal gut flora to do its dirty work, a new study suggests.

A bacterium known as Bacillus thuringiensis produces a toxin that kills a variety of insects, including moths and mosquitoes. Despite the toxin's popularity as an insecticide, the mechanism by which it kills insects was unknown. Jo Handelsman of the University of Wisconsin-Madison and her colleagues suspected that the insects' normal gut bacteria play a role.

The researchers fed gypsy moths antibiotics that wiped out their normal gut bacteria. When those insects received the B. thuringiensis toxin, few of them died.

However, when the researchers gradually replaced the insects' normal gut bacteria, the toxin became more lethal. The researchers found that a strain of Enterobacter microbes seem responsible for turning the toxin into a killer. Further investigation showed that this gut microbe thrives and multiplies when it enters an insect's hemolymph, the equivalent of blood, the researchers write in a study released early online by the Proceedings of the National Academy of Sciences.

These results suggest that the toxin might poke holes in insects' guts, explains Handelsman, allowing Enterobacter and other infectious gut bacteria to flood out into the hemolymph and take over the rest of a bug's body. --CB