Aedes aegypti mosquitoes, carriers of many devastating diseases including dengue, Zika, and yellow fever, thrive in a variety of environments. While particularly pesky in jungles and forests, they even flourish in cities, laying eggs in nutrient-poor pools of standing rainwater. This has left scientists wondering: how is it that they survive so well in urban areas?
A preprint published on February 23 in bioRxiv shows that female mosquitoes might be able to sculpt the environment where they lay their eggs, depositing growth-boosting bacteria in the water alongside their young. The researchers say that the findings might help us develop new ways to control the disease-bearing pests.
“We’ve always wondered how mosquitoes manage to get enough nutrients from such a poor environment,” says study author Marcelo Lorenzo, a biologist at the René Rachou Institute in Minas Gerais, Brazil.
Previous research has shown that adult mosquitoes have a symbiotic relationship with some types of bacteria, which help the insects digest sugars and produce more eggs. Pregnant females are also choosy about the pools in which they deposit their young—they lay their eggs in water that smells like certain bacteria, indicating that some strains might help the young survive. Lorenzo and his colleagues thought mosquitoes might be able to transfer these helpful bacteria into water sources that don’t have them, which Lorenzo likens to “preparing a place where the kids will grow.”
“It’s a really cool study and . . . an important topic,” says Jeff Riffell, a neuroecologist at the University of Washington who was not involved in the work. It raises another question, he adds: “Is there intergenerational transmission in the microbes that could really impact [mosquitoes’] natural history lifespan?”
Mosquito moms’ bacteria of choice
The researchers first tested if pregnant female Aedes aegypti mosquitoes could deposit the bacteria they already carried on their bodies onto other substrates by having the insects land on sterilized food plates. Mosquitoes transferred bacteria primarily from three genuses: Bacillus, Elizabethkingia, and Serratia.
The team then wanted to test which bacterial strains, if any, mosquitoes deposited into the water alongside eggs. To do this, the researchers placed a cup of water filled with fish food inside a sterile cardboard box. They then ran four different experiments: they introduced either sterilized eggs or unsterilized eggs into the water, or a lone, hungry female mosquito that was either pregnant or not into the box. When they cultured and genetically sequenced the bacteria in the water, they found that bacteria grew in all conditions but the diversity of the bacteria decreased significantly after pregnant females laid their eggs in the water (the diversity was similar in the other conditions). They saw that this decrease in bacterial diversity correlated with the presence of Elizabethkingia.
“This Elizabethkingia was a bacteria that we found in our . . . transmission experiments. And we also found that it is an indicator species” of egg laying, Katherine Mosquera, a molecular biologist at the René Rachou Institute and a coauthor of the study, tells The Scientist.
By extracting Elizabethkingia from the guts of mosquitoes collected in the field in Rio de Janeiro and then growing eggs in its presence, the researchers found that the bacteria speed up larval development, decreasing the time it takes mosquitoes to hatch and reach adulthood from 180 hours to 171 hours. By hastening larval growth, the bacteria could accelerate mosquitoes’ life cycles and allow populations to grow faster.
The researchers don’t yet know how Elizabethkingia speed up larval development but predict that larvae and the bacteria have a mutualistic relationship. As the larvae grow more quickly, the scientists speculate they may also help Elizabethkingia snuff out bacterial competitors. When the mosquitoes reach adulthood, previous studies have suggested that the bacteria help mosquitoes transport sugar, break down blood cells, and lay more eggs. The researchers don’t know how Elizabethkingia clears out other bacteria after mosquitoes introduce it into water but speculate that its ability to survive may stem from how “[it] is resistant to several antibiotics,” says Mosquera, which may help it survive antibiotics released by bacterial or fungal competitors.
Lily Khadempour, a Rutgers University ecologist who was not involved in the study, tells The Scientist that she thought the study was good and that “what they’re getting at here is that potentially the bacteria in the system, the Elizabethkingia, could be serving as a defensive symbiotic for the developing larvae.” She says that such mutualistic relationships have been seen previously in other insects such as the beewolf and fungus-growing ants. In both cases certain microbes help their hosts fight off other invaders. “Microbial symbionts associated with insects can also help detoxify environments,” Khadempour adds.
The research team suggests that the mosquitoes’ attraction to Elizabethkingia could be exploited as a means of pest control. Another bacterial strain, Wolbachia, is currently used as a means of preventing mosquitoes from spreading disease. When mosquitoes carry Wolbachia, the bacteria compete with viruses like dengue and Zika. In many locations, including Brazil, where the study was conducted, scientists are attempting to infect A. aegypti, which don’t normally carry Wolbachia, with the bacteria.
The researchers say that Elizabethkingia could be used similarly to Wolbachia, but in this case, as a component of ovitraps, environments that promote egg-laying in enticing but deadly conditions.
Containment strategies exploiting the mutualistic relationship between Elizabethkingia and mosquitoes could complement other mosquito control strategies. “We cannot use only one tool to control mosquitoes; it has to be integrated control initiative with several strategies,” says biologist and Wolbachia researcher Luciano Moreira, an author of the study. Mosquitos can adapt to insecticide exposure and in many locations, mosquito populations have developed resistance to many common pesticides. “Insecticides will not be enough to control mosquitoes,” Moreira adds.
“This will not be a silver bullet. I don’t think we will ever have a silver bullet. We know evolution finds its ways. . . . We need to have integrated pest management,” Lorenzo says.