Engineered honeybee gut bacteria trick attackers into self-destructing

Deadly, fat-sucking mites and wing-wrecking viruses, take note. Specially engineered gut microbes can defend honeybees by tricking their enemies into self-destruction.

Rod-shaped Snodgrassella bacteria, common in bee guts, were engineered to release double-stranded RNA molecules that dial down gene activity in a mite or virus. The pest then sabotages itself by shutting down some of its own vital genes. This strategy hijacks a natural biological process called RNA interference, or RNAi (SN: 10/4/06). The gut bacteria churning out this targeted disinformation work “something like a living vaccine,” says microbiologist Sean Leonard of the University of Texas at Austin.

The RNA’s targeted approach intrigues scientists interested in fighting pests or other problems while minimizing the chances of hurting innocent bystanders.

Earlier work shows that directly dosing bees with the customized RNA also can work, Leonard says, but the stuff is expensive to make and degrades rapidly. A gut microbe, however, can keep making the RNA, replenishing the supply.

In a simplified test, Leonard and colleagues targeted two of the big threats to honeybees in North America: fat-sucking, parasitic Varroa mites and the deformed wing virus that those mites spread among bees (SN: 1/18/19). In a setup with just young bees, the engineered gut microbes helped protect the bees, the scientists report in the Jan. 31 Science.

For the mite test, the researchers tracked fates of the pests. (Collecting mites to spread among experimental bees is easy, Leonard says. Just find infested bees and dust them with powdered sugar. Mites drop off in an arthropod shower.) Mites were about 70 percent more likely to die within 10 days when feeding on bees with the booby-trapped gut microbes.

Virus tests looked promising, too. Bees dosed with protective bacteria had a 37 percent higher survival rate 10 days after exposure to deformed wing virus.

This experiment is a proof of principle, Leonard says. Honeybees don’t really live as they did in the test — in little cuplike cages of 20 equally youthful pals. This gut-microbe technique would need to work in the complexity of a full hive. And the protective bacteria would also need to work within a full bee gut microbiome, the collection of bacteria and other microbes found in the insects’ innards.

“Bees have this remarkably consistent and conserved microbiome,” despite the upheaval of metamorphosis, Leonard says. When a bee larva transforms into an adult, it loses its old gut lining and the microbes that lived there. The newly adult bees replenish their microbiome from hive mates. Normally five kinds of bacteria show up again and again, including the Snodgrassella bacteria engineered for this test.

Harnessing those bacteria to supply the double-stranded RNA is “a really novel and cool way to deliver this system,” says honeybee epidemiologist Dennis vanEngelsdorp of the University of Maryland in College Park.

But he cautions that actual use is a long way off. Besides the inevitable pitfalls in trying to scale up a small lab test, he sees some big questions to consider.  With RNAi, “you’re turning off genes, and there has to be a very healthy debate about how do we regulate this?” he says.  

Using gut bacteria to get such a strong effect on the pests is “a very big deal,” says entomologist Jay Evans at the U.S. Department of Agriculture’s Bee Research Laboratory in Beltsville, Md. “We have tried with mixed results for years, probably because the other forms of [RNA] delivery are poor.” But whether the world is ready for bees with genetically engineered gut microbes is another thing. He doesn’t expect these bees to be buzzing through almond groves or apple orchards anytime soon.