Gut bacteria ally with Bt

Caterpillars shouldn’t always trust their guts. A new study reports that stomach microbes can betray their pest hosts, conspiring with Bt toxin to kill the lepidopteran larvae.

The findings, released online March 3 in BMC Biology, reveal “a new realm of exciting interactions that we didn’t anticipate,” comments entomologist Bruce Tabashnik of the University of Arizona in Tucson.

A greater understanding of the interactions among the toxin, the pests it kills and their gut microbes may lead to designer pest control regimes, Tabashnik says.  “Bt proteins are a great tool for pest control because they are so specific,” he says. “If we know more about the insects, we can be even wiser about how we use them [Bt proteins].”

Nichole Broderick, who conducted the research as part of her Ph.D. research at the University of Wisconsin–Madison, says she set out investigating the opposite notion: that gut bacteria provide caterpillar hosts with protection against Bt.

Bacillus thuringiensis is a common soil bacterium and has become a widely used insecticide since its discovery more than a century ago. As part of its life cycle it makes crystalline toxins that are lethal to particular insect groups. Some Bt toxins work best against beetles, others against flies. But the most widely used are those that kill caterpillar pests such as the European corn borer. Heralded for inflicting harm with such specificity, genes from the bacterium have been spliced into some crops. Worldwide 42 million hectares of Bt corn and cotton, an area nearly the size of California, were planted in 2007.

While many Bt genes have been identified and cloned, how the toxin delivers its blow isn’t precisely clear. Butterfly and moth larvae (caterpillars) have very alkaline guts that dissolve the crystalline Bt protein, yielding a protoxin. This protoxin is split into a smaller protein known as activated toxin, which then binds to specific gut receptors. Holes then form in the gut membrane leading to insect death. So far, investigations of insect resistance to Bt point to genetic mutations that truncate gut receptors, not a lack of the offending gut microbes.

“Insects die by more than one mechanism,” Broderick says. “We still don’t know much about the events that lead to death.”

Now it seems the insects’ normally neutral gut bacteria play a role. Caterpillars typically have only six to eight species of microbe in their guts, versus the great diversity found in termites or mammals. Broderick and colleagues had previously discovered that gypsy moth larvae become invincible to Bt in the absence of gypsy moth gut bacteria.

In the new study, the researchers investigated six butterfly and moth species. To clear the guts of microbes, the researchers reared the young caterpillars on antibiotics. In five of the species the Bt toxin was harmless against the microbe-free caterpillars. When the researchers inoculated the caterpillars’ guts with the microbe Enterobacter, found in the gut of gypsy moth caterpillars, the Bt toxin became effective again in four of the five species.

The only caterpillar that remained resistant typically harbors bacteria that have dramatically different outer membranes than species harbored by other larvae.

How the microbes work in concert with Bt isn’t clear. Indeed, the research raises more questions than it answers, says Kenneth Raffa, Broderick’s thesis adviser at the University of Wisconsin.