Gut bacteria of Aedes aegypti could become a weapon against dengue
AGENCIA FAPESP/DICYT Understanding how the bacteria that colonize the gut of Aedes aegypti influence the mosquito’s susceptibility to the dengue virus is the goal of a research project being conducted at the Botucatu campus of São Paulo State University (UNESP) with FAPESP’s support.
According to Jayme Augusto de Souza-Neto, lead investigator of the project, the discovery of factors that make the mosquito resistant to the virus could pave the way to future development of strategies to block transmission of the disease.
“We aim to discover how information is exchanged between the mosquito’s gut microbiota and its immune system. We want to understand how this allows for the successful infection of the insect and consequent transmission of the virus to humans,” Souza-Neto said.
When the Aedes aegypti mosquito sucks contaminated blood, he explained, the pathogen first lodges and replicates in the gut. Once lodged in this tissue, it must circumvent the organism’s defense mechanisms to be able to travel through the insect’s body until it reaches the salivary glands.
“One of its main defense mechanisms is the activation of genes that encode antiviral proteins,” Souza-Neto said. “The gut microbiota is another important component, but we don’t yet know exactly how it influences the immune response.”
Some bacteria may act directly by producing molecules with antiviral activity. Others may indirectly interfere by activating signaling pathways that stimulate the insect’s immune system to combat the invader.
Previous research has shown that dengue virus susceptibility varies considerably in different populations of the vector mosquito. The initial findings of experiments performed by Souza-Neto’s group at UNESP’s Biotechnology Institute (IBTEC) suggest that the gut microbiota is one of the factors behind this variability.
The researchers compared a population of A. aegypti collected in the municipality of Botucatu, where the incidence of dengue is low despite the constant presence of the mosquito, with a population collected in the city of Neópolis, Sergipe State, where many cases of the disease are reported every year.
In the laboratory, both groups of mosquitos were fed with contaminated blood and were subjected to tests to quantify viral load in the organisms four days later. The infection rate was in the range of 25% in the mosquitos from Botucatu, whereas it was more than 80% in the group from Neópolis.
Using large-scale genetic sequencing techniques, the researchers identified all of the species of bacteria that had colonized the mosquitoes’ guts and observed that the two groups had completely different microbiomes.
In the insects from Botucatu, the predominant classes were Flavoproteobacteria and Gammaproteobacteria, which were found in similar proportions in both infected and non-infected mosquitos. The predominant genera were Elizabethkingia and Asaia.
In the Neópolis group, the most common class was Gammaproteobacteria, with the genera Pseudomonas and Stenotrophomonas predominating in infected and non-infected mosquitoes, respectively.
The analysis also showed that contact with the virus induced more significant activation of key antiviral defense genes in the population from Neópolis, Sergipe. “Roughly a third of the genes activated in the mosquitoes from Sergipe are controlled by immune system pathways associated with the control of bacteria,” Souza-Neto said. “They are genes that encode antimicrobial peptides such as defensins, which reinforces the hypothesis that the microbiome participates in this integrated anti-dengue system in the mosquito’s gut.”
According to Souza-Neto, many of the bacteria identified in both populations of A. aegypti are species commonly found in soil, indicating that the environment in which the mosquito develops influences the microbiota composition during the insect’s adult phase.
The experiments were performed as part of Carine Spenassatto Dreyer’s doctoral research, supported by a scholarship from FAPESP.
The researchers are currently trying to separate the genes that are activated by infection independently of the microbiota from those that undergo interference by gut bacteria.
Although the research project being conducted at IBTEC-UNESP is designed only to gain an understanding of the complex interaction between the mosquito’s gut microbiome and its immune response to the dengue virus, Souza-Neto envisions opportunities for future application of this knowledge.
“If we can find a bacterial species capable of inducing a strong antiviral response in the mosquito, we will open up the possibility of cultivating populations of A. aegypti in which that bacterium is predominant in the gut microbiome,” he said.
The same concept could be adapted if a bacterial species capable of producing antiviral molecules was found. In both cases, laboratory-grown populations would theoretically be resistant to the virus and could block the chain of disease transmission if they were released into the wild in large quantities.
Another possibility, according to Souza-Neto, would be to find a key gene that makes the insect resistant to infection. “In this case, it would be possible to develop a transgenic mosquito in which this gene is overexpressed,” he said.
The first genetically engineered A. aegypti mosquitoes were unveiled in Brazil in 2014 by Oxitec, a UK-based company. In this case, however, the genetic modification does not alter the insect’s ability to defend itself from the disease but rather prevents it from producing viable progeny.
In Souza-Neto’s view, the two strategies are complementary, similar to all other vector control measures, including the elimination of breeding grounds.
“It’s important for people not to interpret scientific innovations as solutions to all of their problems,” he said. “All of the strategies must work together in an integrated manner. None of them can replace any of the others.”