Sometimes scientists get it wrong and have to revise the conventional wisdom they helped create. A bacterium known as Bacillus thuringiensis produces a toxic crystal that has been identified as a killer of garden pests since at least 1911. Researched extensively, the so-called Bt toxin was turned into a commercial product starting in the 1950s, and more recently, plants have been engineered to produce it on their own. Yet, through all these years of research, the mechanism by which it kills bugs has simply been assumed: the toxin eats holes in the insect's gut and they either starve or experience septicemia--a deadly bacterial infection in the blood--from the Bt itself. "If you don't have a good gut wall, you can't take up nutrients," explains microbiologist Jo Handelsman of the University of Wisconsin-Madison.
But Wisconsin graduate student Nichole Broderick discovered that nearly a century's worth of consensus was wrong when she cleared gypsy moth caterpillars of the bacteria in their gut and fed them Bt toxin. "Initially, I was testing the hypothesis that the gut bacteria were actually protecting the moth," she says. "I found that once they did not have a gut community [of bacteria] I could no longer kill them with Bt." In fact, large numbers of the nascent moths did just fine--even when fed as much as 10 international units of the toxin, 10 times more than ordinarily needed--as long as they were also fed antibiotics that cleared their systems of the seven to 20 bacteria species their guts normally host. "I think it was by the third fairly large, replicated experiment that we looked at each other and said, 'this is real,'" Handelsman recalls. The Bt toxin could not kill on its own.
Reintroducing bacterium one-by-one revealed that Enterobacter species NAB3 reinstated Bt's deadly impact. Further tests with insect blood showed that this Enterobacter thrived in the hemolymph, multiplying as much as when raised in rich laboratory broth, whereas B. thuringiensis was cleared from the blood within six hours. And introducing Escherichia coli capable of producing Bt toxin quickly killed larvae, unless the E. coli was heat-killed and introduced into caterpillars without any other gut bacteria. "The gut wall will heal and regenerate itself if there are no gut bacteria there," Handelsman says.
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Of course, none of this answers the basic question: how does Bt toxin, in partnership or alone, kill insects? "Is the Enterobacter moving into the blood? At this point, that's the hypothesis we're pursuing," Broderick notes. "I think it's probably more complex than that. The simple model doesn't always turn out to be the case." As her prior work proves once again. The paper presenting the finding publishes online in Proceedings of the National Academy of Sciences USA on September 26.
