Bacteria engineered to protect bees from colony collapse

Wednesday, 01 April, 2020

Bacteria engineered to protect bees from colony collapse

US scientists have developed a new strategy to protect honey bees from the deadly trend known as colony collapse: genetically engineered strains of bacteria.

Honey bees play an enormous role in global food production — without them, dozens of crops, from almonds to berries to broccoli, would either vanish or produce significantly less food. But honey bee numbers have been dwindling for the past several years, with a US survey revealing that beekeepers lost nearly 40% of their honey bee colonies last winter — the highest rate reported since the survey began 13 years ago. Two major causes of colony collapse are Varroa mites and deformed wing virus, which unfortunately often come as a pair; as the mites feed on bees, they can spread the virus, while also weakening the bees and making them more vulnerable to pathogens in the environment.

Seeking to combat this problem, scientists from The University of Texas at Austin engineered bacteria to live in the guts of honey bees and act as biological factories, pumping out medicines protecting the bees against both Varroa mites and deformed wing virus. Their work was led by primary investigator Professor Nancy Moran and has been reported in the journal Science.

A Varroa mite, a common pest that can weaken bees and make them more susceptible to pathogens, feeds on a honey bee. Image credit: Alex Wild/University of Texas at Austin.

Like humans, honey bees have an ecosystem of bacteria in their guts called a microbiome and also an antiviral defence mechanism called RNA interference (RNAi) that helps the body fight off certain viruses, called RNA viruses. When an RNA virus is introduced, it produces molecules called double-stranded RNAs that a healthy cell detects, triggering an RNAi immune response.

“You usually only get signs of these molecules when an RNA virus is replicating,” Prof Moran said. “It’s a signal that this might be an evil thing and you should attack it.”

To promote a helpful RNAi response to viruses in bees, and trigger a lethal RNAi response in the mites, the team introduced modified bacteria to hundreds of bees in a laboratory setting — one strain engineered to target the virus and another for the mites. Sprayed with a sugar water solution containing the bacteria, the bees groomed one another and ingested the solution.

The team found inoculating young worker bees with the engineered bacteria led the bees’ immune systems to be primed to protect them against deformed wing virus, which is an RNA virus, and caused the mites’ own immune systems to fight against and ultimately kill them. Compared with control bees, the bees treated with the strain of bacteria targeting the virus were 36.5% more likely to survive to day 10. Meanwhile, Varroa mites feeding on another set of bees treated with the mite-targeting strain of bacteria were about 70% more likely to die by day 10 than mites feeding on control bees.

“This is the first time anyone has improved the health of bees by genetically engineering their microbiome,” said graduate student Sean Leonard, first author of the study.

Image credit: University of Texas at Austin.

Another benefit of the approach is that researchers can use it as a tool in studying bee genetics. The engineered bacteria can knock down specific bee genes, enabling insights into the workings of the bee genome and possibly enabling new breeding strategies to produce more robust bee colonies.

The researchers believe their method could one day scale up for agricultural use because the engineered bacteria are easy to grow, inoculating the bees is straightforward and the engineered bacteria are unlikely to spread beyond bees. This is because the type of bacteria used are highly specialised to live in the bee gut, can’t survive for long outside of it and are protective for a virus that strikes only bees. That said, further research will be needed to determine the effectiveness and safety of the treatments in agricultural settings.

Top image caption: Prof Moran and her team raise thousands of bees for their research in rooftop hives on The University of Texas at Austin campus. Image credit: Marsha Miller/University of Texas at Austin.

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