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"It was one of those assumptions built on assumptions -- a scientific house of cards," explains one of the report's authors, Jo Handelsman, of the long-held view of Bt's mode of killing. "What was proposed as a hypothesis in one paper became cited as proven in another and no one seemed to go back to the original literature until now."

Handelsman is a Howard Hughes Medical Institute Professor in the UW-Madison department of plant pathology.

The new work, conducted in the laboratories of Handelsman and Kenneth F. Raffa, a professor in the UW-Madison department of entomology, demonstrates that Bt requires the presence of other bacteria to exert its lethal influence.

Virtually all animals, including humans, depend on the interplay of numerous species of bacteria that, beginning at birth, routinely colonize the stomach and intestines. The caterpillars of moths and butterflies, for example, have anywhere from seven to twenty species of gut bacteria. Humans have between five hundred and one thousand species of micro flora that take up residence in the intestinal tract.

"In moths and butterflies, the complexity is much lower than in mammals, and even some other insects," Broderick explains.

The Wisconsin study was conducted using antibiotics to clear all of the native bacteria that colonize the gut of gypsy moth caterpillars. Exposed to Bt, the caterpillars whose intestinal tracts had been cleared of their native microbial communities showed none of the agent's toxic effects.

When the insect's microbial gut flora were reestablished, Bt's insecticidal activity was restored.To further test their results, the Wisconsin team used a strain of live E. coli engineered to carry the Bt toxin to infect caterpillars, a lethal treatment whether or not the insect gut contained its normal complement of microbes. However, if the engineered E. coli was killed before administration, it
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Source:University of Wisconsin-Madison