The researchers evaluated populations of Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus collected both in the field and raised in a lab setting to determine whether or not the mosquitoes required bacterial populations in their guts to develop, and which bacteria were necessary to the process.
A team of entomologists from the University of Georgia has determined that certain species of mosquitoes, including the species that carries Yellow Fever, Dengue fever, and the Zika virus, host bacterial populations in their gut that are essential for the insects’ development.
Given that the Centers for Disease Control and Prevention estimates that the Ae. aegypti and albopictus mosquitos are spreading north and westward throughout the United States each year— bringing with them the lingering threat of transmission of diseases like Dengue, yellow fever, and Zika virus— controlling mosquito populations could be vital to minimizing and controlling health threats in the future.
The researchers evaluated populations of Aedes aegypti, Aedes albopictus, and Culex quinquefasciatus both collected in the field and raised in a lab setting to determine whether or not the mosquitoes required bacterial populations in their guts to develop, and which bacteria were necessary to the process. The group determined that all three species of mosquito did require the presence of bacterial populations in order to develop but that they did not rely on one particular bacteria. They believe that this is probably “because larvae do not reliably encounter the same bacteria in the aquatic habitats they develop in,” the team stated. They also determined that mosquito larvae gain these bacterial communities by consuming them from the environment, not by inheriting them.
In order to evaluate mosquito development, the team used rRNA gene analysis to identify different types of bacteria in the mosquitoes’ guts and compare the populations within and between species depending on whether or not the mosquitoes were raised in a laboratory setting or collected in the wild. During the process, they noted that the antibiotic treatments used to eliminate the bacteria from the larvae did not fully eliminate the bacterial populations, resulting in mosquito larvae that were still able to reach full maturity. However, “high concentrations of some antibiotics [did] adversely affect larvae independent of their impact on bacteria,” the researchers noted.
“For us, this was important because antibiotics are used as a means of limiting bacterial infections in lab populations and, as it turns out, it is not a very useful thing to do when dealing with field populations of mosquitos because there are pretty high levels of resistance,” explained Michael Strand, a Regents Professor of entomology at the University of Georgia. However, he added, there is a wider implication to this particular finding as well. “This is another example of how natural populations of bacteria can have fairly high resistance to common antibiotics. There may be higher levels of resistance than one might expect in natural communities where humans exist, probably because of heavy use of antibiotics in all different kinds of applications,” he said.
The team identified a total of 29 bacterial phyla in the population samples; however, six— Proteobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Cyanobacteria, and Verrucomicrobia– accounted for 81% of the populations in the samples. Of particular note was one Ae. aegypti population that had a dual infection of developmentally-relevant gut bacteria and Wolbachia, which is not known to have naturally infected this species anywhere in the world. Wolbachia may reduce mosquitoes’ ability to transmit Dengue, Zika, chikungunya, and yellow fever, but not all strains of the bacterium have this ability. “A number of groups worldwide are using this bacterium as a way to make mosquitos nonpermissive to infection by disease agents like Dengue and Zika,” said Dr. Strand. “If a mosquito is already infected with one strain of Wolbachia, then it could potentially affect the introduction of a new strain for disease management,” he added.
After identifying all bacterial infections in the mosquito populations and confirming that mosquito larvae would not survive to maturity without those infections, the team assessed how the bacterial populations themselves responded to antibiotic treatments intended to eliminate the bacteria. Conventionally, ampicillin, kanamycin, streptomycin, and penicillin are used to eliminate bacteria in this manner. “The bacteria exhibited highly variable sensitivities,” the team noted, adding, “In some cases, [bacteria] were largely unaffected at high doses.”
The team focused on Ae. aegypti for antibiotic testing, and although not all larvae survived antibiotic treatments, those that did continued to develop. This indicated that the bacterial communities the larvae hosted did have some antibiotic resistance. At high dosages of antibiotics, the larvae died whether or not they had bacterial infections.
“Our long-term goal is to understand the underlying mechanisms for why insects need gut microbial populations to grow,” said Dr. Strand. He added, “These populations could be a means of potentially preventing mosquitos from growing into adults and spreading diseases, which has real practical significance for controlling and managing mosquito populations.”