Bacteria works as a barrier against infection and reduces viral load in flies, study finds

A study conducted by researchers from the universities of São Paulo (USP) and Cambridge (United Kingdom) showed that flies of the Drosophila melanogaster species — common in any kitchen — are infected by fewer virus species and contain a lower viral load when they are colonized by bacteria. of the Wolbachia genus.

The work, supported by Fapesp and the Royal Society, was published in the journal Communications Biology.

The results reinforce the hypothesis that Drosophila and Wolbachia live in a relationship of mutualism, in which both benefit, and not from parasitism, as has been suspected. The finding may have repercussions in the fight against diseases transmitted by other insects.

“Wolbachia is present in more than half of terrestrial insects. For a long time, it was thought that it was a reproductive parasite, because there are some lineages in which it alters the reproduction of the host and, with that, increases its frequency in the population. But with the advances in molecular biology of the last two decades, it was discovered that in many cases it does not do this manipulation of reproduction. The only way to explain why it is so common, then, would be to bring some benefit”, he says. Rodrigo Cogni, professor at the Institute of Biosciences (IB-USP) and first author of the article.

Wolbachia has been identified before as having a protective action against viruses. However, the experiments were carried out with animals under laboratory conditions. For example, one could inoculate the bacteria into previously uncolonized flies and compare the survival rate after viral infection with that of flies that did not have the microorganism.

This is the first time that the presence of Wolbachia and its antiviral action has been verified in Drosophila melanogaster collected in the field, therefore, showing that the relationship exists in natural populations.

The bacterium is already used, including in actions to combat viral diseases transmitted by mosquitoes, such as dengue, zika and chikungunya. Studies in Australia and in Asian countries showed that regions where Aedes aegypti contaminated in the laboratory with Wolbachia were released had fewer cases of these diseases than in areas without this type of intervention.

The premise of these interventions is that, as the bacterium is transmitted from the female to the eggs, mosquitoes with Wolbachia end up predominating in the population and the replication of the virus is blocked in the insects, preventing the infection of humans.

The use of Wolbito, as the mosquito with the bacteria is called, is an action carried out by the World Mosquito Program since 2011. In Brazil, it is currently carried out in five municipalities by Fiocruz, with support from the Ministry of Health.

The current study contributes to a better understanding of the relationship between bacteria and insects and can serve as a subsidy to improve actions like this.

protected flies

The researchers collected 1,014 flies of the species D. melanogaster in an orchard in Connecticut, United States. The bacterium was present in 71% of the individuals.

Using genetic sequencing techniques, the researchers obtained the total RNA expressed in each fly. The vast majority (93%) had at least one virus. Thirty of them were identified, 17 known to infect D. melanogaster and 13 new ones, related to other insect viruses or even fungi and trypanosomes.

“As viruses can be present in the food, environment and pathogens of flies, we cannot guarantee that all of them necessarily infect them. However, since many are related to other arthropod viruses, we suggest that some of them should, yes. , infect them”, says Cogni.

Flies that had Wolbachia had an average of 15% less virus than those free of the bacteria. In addition, the viral load — the amount of live virus in the body — was lower in those carrying the bacteria.

“Previous studies from other groups indicate that Wolbachia has to be mutualistic, or it would not be present at such a high frequency. We are now showing that it exerts protection in natural populations, which reinforces the hypothesis of mutualism. However, this does not mean that that there is no cost for the flies to harbor the bacteria”, informs the researcher.

Other studies have already shown that, if, on the one hand, the higher density of Wolbachia increases protection against viruses, on the other hand, it increases energy expenditure, resulting, for example, in lower egg production. From what the study indicates, it’s good value for money.