By Erin Weeks
Imagine that a strain of bacteria living in your body had the power to protect you from deadly viruses, render you sterile, or even help give rise to a new species of human. For insects, it’s not just fantasy — one widespread variety of bacteria possesses all of these powers, and many more bizarre qualities. Scientists across the globe are now learning to commandeer those bacteria for human benefit in hopes of combating disease-spreading mosquitoes. And judging from recent efforts in Kentucky, the future of our partnership with the microbe Wolbachia looks bright.
Wolbachia is a genus of bacteria that can live within the cells of anywhere from 20-70% of the world’s insects, as well as many related arthropods. In many cases, the bacterium hangs in symbiotic balance in an insect’s cells as a neutral or even beneficial guest to its host. But since the mid-twentieth century, scientists have discovered that Wolbachia can also wreak havoc on host organisms, particularly on their sex lives.
In the common pill-bug, Wolbachia can have a feminizing effect, turning males into females. In some mites, the bacteria induce parthenogenesis, a process by which females breed asexually without the genetic contribution of males. In mosquitoes, males and females infected with Wolbachia must possess the same strain to successfully reproduce — if there’s a mismatch, the bacteria will sabotage the male’s sperm, effectively sterilizing him.
It’s this last phenomenon that Dr. Stephen Dobson studies in his lab at the University of Kentucky. Dobson’s research has long focused on Wolbachia’s impact on individual insects and their populations, but in recent years his team has made headlines in their quest to hijack the bacteria in the war against mosquitoes.
Mosquitoes transmit a staggering variety of life-threatening pathogens — malaria, dengue, chikungunya, yellow fever, and West Nile virus, among others — that infect hundreds of millions of people each year. These numbers could accelerate with climate change and global trade, which have already facilitated the spread of highly aggressive and invasive species such as the Asian tiger mosquito (Aedes albopictus). Therefore, identifying new methods of mosquito control remains a high priority for global health organizations.
That’s where Dobson’s research comes in, by exploiting the natural inability of mosquitoes harboring different Wolbachia strains to produce offspring.
“Wolbachia is able to ‘poison’ the sperm,” Dobson said. “When those sperm fertilize an egg, the sperm essentially self destructs.”
This process is called cytoplasmic incompatibility, and despite the prevalence of Wolbachia, the precise molecular mechanisms behind it largely remain a mystery. A peek into the sex cells of mated female mosquitoes does provide some clues, however. A normally developing embryo receives a set of chromosomes from each parent. In an egg fertilized by sperm with an incompatible Wolbachia strain, the male chromosomes never form properly. Instead, they clump into a mass of genetic spaghetti, and the embryo dies.
Dobson and a former student founded the company MosquitoMate to turn their lab discoveries into a commercial technology for mosquito control. The process they developed works like this: The researchers first identify the Wolbachia strain with which local female mosquitoes are infected. Next, male mosquito embryos in the lab are injected with a different strain, often one generated artificially by treating a wild strain with antibiotics. When the lab-grown adult males are released into the local environment, they seek out and mate with the females, but their offspring are doomed — the incompatible Wolbachia strains kill off the embryos.
“There’s no known resistance to Wolbachia, and it’s species-specific,” Dobson said of the technology’s advantages over traditional chemical control. “And instead of relying on the wind to deliver the insecticide, we’re using insects as the delivery vehicle.”
The natural environment is full of cracks and holes where chemicals may never reach hiding mosquitoes, but male mosquitoes in search of mates are able to hunt down elusive females.
Teams in other parts of the world are similarly working to subvert mosquito biology in the service of disease control. In Australia, the Eliminate Dengue research program relies on another quirk of Wolbachia — its capacity to protect host insects from viruses. Scientists have found that certain strains of Wolbachia not only reduce the likelihood that host mosquitoes will become infected with pathogens, but also limit their ability to transmit those pathogens. In other words, Wolbachia-infected mosquitoes are less likely to pass on diseases such as dengue to humans.
The Eliminate Dengue program releases Wolbachia-positive female mosquitoes into the environment with the hope of eventually replacing the local gene pool with mosquitoes unable to transmit dengue. After initial small-scale success in the Cairns region of Australia, similar projects are now being rolled out in Brazil, Colombia, Indonesia, and Vietnam.
Stephen Dobson and his team began their first field trials last year in Lexington, Kentucky with the blessing of the U.S. Environmental Protection Agency (EPA).
“We are encouraged by the initial results,” Dobson said of the trial so far.
His team’s data is not yet public, but Dobson says adult population sizes in testing areas have been promising. In the meantime, they’re moving ahead on testing the technology in other locales across the United States. If they can show the Wolbachia-sterilized males to be effective at suppressing population levels in different environments, the EPA will consider licensing the technology for widespread use.
Rather than replacing existing methods, Dobson envisions the technology becoming another tool in the mosquito-control arsenal.
“We’re still going to need chemical pesticides,” he said. “An integrated approach in reality is going to be the most powerful. But by introducing this, we can potentially reduce the amount of chemicals used.”