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This brown mite (center) often infects honey bees, transmitting deadly viruses to them.

Alexander Wild

Mite-destroying gut bacterium might help save vulnerable honey bees

The world’s honey bees are facing an unprecedented crisis. Since the 1940s, the number of honey bee hives in the United States has dropped from 6 million to 2.5 million. A combination of colony-killing mites, viral pathogens, and possibly pesticides is largely to blame. Now, researchers are tapping an unusual ally in the fight to bring the bees back: a bacterium that lives solely in their guts. By genetically modifying the bacterium to trick the mite or a virus to destroy some of its own DNA, scientists have improved bee survival in the lab—and killed many of the mites that were parasitizing the insects.

The work, which has yet to be tested in whole hives or outdoors, promises to be effective over the long term, says Robert Paxton, a bee ecologist at Martin Luther University, Halle-Wittenberg, who was not involved with the study. It could help end, he says, “the major plagues of the honey bee.”

Those plagues include the aptly named Varroa destructor mite, which weakens bees by feeding on their fat stores, as well as the deadly “deformed wing” virus the mite transmits when it makes its home on the bees’ bodies. All too quickly, the mites have developed resistance to pesticides that used to kill them, Paxton says.

To bypass the pesticides, some scientists have turned to a process called RNA interference. RNA is best known for transferring DNA’s protein-coding messages to a cell’s protein-production machinery. But RNA can also be recruited to help “silence” unwelcome genetic material. By engineering RNA to match the sequence of the undesirable gene, scientists can activate the cell’s ability to shut down the matching genes, even those that lead to disease in humans.

Jeffrey Barrick, a microbial evolutionary biologist at the University of Texas, Austin, and his colleagues decided to see whether they could recruit bacteria living in the honey bee gut to produce RNA that make the mite—or the virus—dismantle some of its own genes. Whereas humans have thousands of kinds of gut bacteria (and no two humans have exactly the same set of microbes), all honey bees have the same six to eight gut microbes, which keep the bees healthy. So, if the procedure worked in one set of bees, Barrick reasoned, it could be broadly applied.

Barrick’s graduate student Sean Leonard figured out how to genetically modify one of these bacteria, Snodgrassella alvi, so that it continually made RNA that matched the genetic material he wanted to dismantle: genes that are essential to the survival of the mite or the virus. To watch the RNA diffuse from the honey bee gut throughout the body, he added fluorescent tags.

Next, he fed the bacterium to groups of up to 20 bees before exposing them to the mites or the virus. The mites were 70% more likely to die on the treated bees than untreated ones, Leonard, Barrick, and their colleagues report today in Science. “The mite-killing impact was impressive,” says virologist Michelle Flenniken from Montana State University, who was not involved with the work. When the bees were infected with the virus, they were 36% more likely to survive when they housed gut microbes with virus-targeting RNA than with gut microbes not making RNA, the team reports.

The modified gut bacterium persists in the honey bee’s gut for at least the length of the experiments—15 days—providing a steady supply of antimite and antiviral RNA. And because adult bees feed developing bees, they may be able to transfer these helpful gut microbes to the next generation, Barrick says.

In theory, other RNAs could be added to the microbe to improve bee health and perhaps even make the bees less susceptible to pesticides. “It is a bit like a customized medicine for honey bees,” says Jeffrey Scott, an insect toxicologist at Cornell University in Ithaca who was not involved with the work. “Being able to engineer a gut microbe and specifically regulate gene expression in the host has enormous implications.”

He and others caution, however, that bacteria are typically not easy to contain, raising concerns about using this approach in the wild. Furthermore, much more work needs to be done to establish the effectiveness of the new approach in hives with tens of thousands of bees. But, Paxton says, “If the technique works in the field, that could be the end of Varroa and the viruses.” At least until these pathogens develop resistance.