Researchers identify bacteria capable of killing Aedes aegypti larvae

Brazilian scientists have identified six species of bacteria with the potential to be used as biolarvicides against the mosquito that transmits dengue, Zika, yellow fever and chikungunya.

By Karina Toledo  |  Agência FAPESP – Researchers at São Paulo State University (UNESP) in Botucatu, Brazil, have identified six species of bacteria with the potential to be used as biolarvicides against the mosquito Aedes aegypti, which transmits dengue, Zika, yellow fever and chikungunya.

Preliminary results of the research, supported by FAPESP, were presented by Jayme Souza-Neto, Head of the Vector Functional Genomics & Microbiology Laboratory (Vectomics) at UNESP’s Institute of Biotechnology (IBTEC), during the second meeting of the ILP-FAPESP Science & Innovation Cycle held on November 27, 2017, at the São Paulo State Legislative Assembly (ALESP).

“We isolated about 30 different bacteria found in the gut of mosquitoes collected in Botucatu and put them each separately in contact with the larvae of these insects. We found that six species of bacteria were capable of killing between 60% and 90% of the larvae, depending on the isolate, in up to 48 hours,” Souza-Neto told Agência FAPESP.

More research is required to characterize the microorganisms’ larvicidal potential in greater detail, he added. This includes estimating the concentration and minimum exposure time necessary for the larvicidal activity and finding out how long the bacteria remain active, among other factors.

“The research is still in the early stages. In the future, we also plan to isolate a number of products released by these bacteria into the external medium in order to understand how they kill these larvae,” said Souza-Neto, who is also a professor in UNESP’s School of Agronomic Sciences. 

Previous research by Souza-Neto’s group showed that A. aegypti mosquitoes found in Botucatu are less susceptible to infection by dengue virus than those found in Neópolis, Sergipe State, and Campo Grande, state capital of Mato Grosso do Sul. The incidence of dengue is higher in both of the latter.

After feeding the mosquitoes in the lab on blood contaminated with dengue virus serotype 4, the group observed that only 30% of those collected in Botucatu became infected, compared with between 70% and 80% in the other two populations.

Using whole-genome sequencing, the group identified the species of bacteria that colonized the insects’ gut and found completely different microbiomes in the more and less susceptible populations.

“We then began investigating the potential of these gut microbiota to act as biolarvicides and also as antivirals,” Souza-Neto said. “In this second run of experiments, we placed the bacteria or the products they released in contact with dengue virus and observed whether the pathogen lost its capacity to infect cells.”

According to Souza-Neto, the same kind of experiment will shortly be performed with Zika virus. “If we succeed in identifying a bacterium that can neutralize these pathogens, it will be a potential source of novel drugs,” he said.

Benefits for society

The aim of the ILP-FAPESP Science & Innovation Cycle, a partnership between ALESP (ILP stands for the São Paulo State Legislative Assembly Institute) and FAPESP, is to disseminate the findings of research with a high social and economic impact produced by scientists in São Paulo State.

In addition to Souza-Neto, the second meeting in the series, entitled “Science in the fight against dengue, Zika and chikungunya”, featured researchers José Luiz Proença Modena from the University of Campinas (UNICAMP), Maurício Lacerda Nogueira from the São José do Rio Preto Medical School (FAMERP), and Alexander Roberto Precioso, director of Butantan Institute’s Clinical Trials & Pharmacovigilance Division. 

Modena’s presentation addressed ongoing studies at UNICAMP’s Biology Institute to characterize key factors for the replication of emerging viruses in both invertebrate and vertebrate hosts.

“By analyzing lipids found in mosquitoes and human patients, we were able to identify biomarkers of infection by Zika,” Modena said. Some of these molecules are important for the virus to enter cells and replicate and are therefore potential therapeutic targets (read more at agencia.fapesp.br/24230).

The UNICAMP group also discovered that children suffering from severe neurological complications associated with congenital infection by Zika virus had low blood levels of hepatocyte growth factor (HGF).

“This opens up the possibility of using molecules that stimulate the production of HGF to prevent neurological damage in the children of mothers exposed to the virus during pregnancy,” Modena said.

He also mentioned the development of a new molecular method of diagnosing Zika, which targets the region of the viral genome that encodes the protein NS5, calling it the most sensitive method of detecting the pathogen in blood a few days after contamination, and a method of diagnosing Zika infection in patient samples by mass spectrometry, which is much less costly than the molecular methods currently available.

“The test that analyzes the viral genome costs about 100 Brazilian reals per patient [currently 30 US dollars], and you have to wait about three days for the result. The method using mass spectrometry costs 1.50 BRL and is ready in 20 minutes,” Modena said.

Nogueira, who also chairs the Brazilian Society for Virology (SBV), warned during his presentation that it is still impossible in Brazil to calculate the real economic impact of dengue and Zika owing to a lack of accurate epidemiological data. Studies conducted at FAMERP showed that the clinical criteria used to diagnose these diseases, based mainly on patient symptomatology, are often faulty.

The team led by Nogueira used molecular testing to analyze blood samples from 800 patients with suspected dengue who visited the emergency room of São José do Rio Preto’s reference hospital between January and August 2016. The initial diagnosis, based on clinical symptoms and serological tests, was confirmed in only 400 samples. 

More than 100 of the cases tested positive for Zika virus, and the virus that causes chikungunya fever was identified in one of the samples. None of the three arboviruses transmitted by A. aegypti was found in the almost 300 remaining samples (read more at agencia.fapesp.br/23858).

Among 400 other patients diagnosed with Zika by clinical and epidemiological criteria, only 20% were found to be infected by the virus in molecular tests, while 11% had dengue and 1% had chikungunya.

“We have a dengue vaccine that’s been licensed and another under development at Butantan Institute,” Nogueira said, “but in order for us to estimate the cost-effectiveness of these vaccines, meaning how much it will cost to vaccinate the population and how much the initiative will save in terms of hospitalizations and deaths, it’s most important to know how many cases of the disease there are.”

The FAMERP group also prospectively monitored a group of 55 women living in São José do Rio Preto with a confirmed diagnosis of Zika virus infection during pregnancy. Approximately 28% of the babies born to these women were found to have some kind of alteration at birth, such as minor brain calcifications, small lesions in brain blood vessels, unilateral hearing loss or retina damage.

The pattern observed in São José do Rio Preto, according to Nogueira, contrasts strongly with that observed in Rio de Janeiro, and the adverse effects of infection were less severe in the former city. All the babies were born alive at term, and no cases of microcephaly or severe neurological alterations were identified (read more at agencia.fapesp.br/25511).

This study also showed that the virus could be detected in the urine of pregnant women for up to seven months. In some cases, the viral load disappeared and later returned, although it was almost always near the detection threshold. According to Nogueira, this suggests a single negative antenatal test result may not be enough to rule out infection by Zika (read more at agencia.fapesp.br/26488).

Finally, Precioso reported that 12 clinical trials of candidate vaccines were performed at Butantan Institute between 2003 and 2017, involving 497 researchers and almost 21,000 volunteers. The largest of these, currently under way at 16 research centers across Brazil, is the Phase 3 trial of a vaccine against all four dengue serotypes (read more at agencia.fapesp.br/22839).

Initial development of the tetravalent vaccine was supported by FAPESP.

During the opening session of the event, FAPESP Vice President Eduardo Moacyr Krieger stressed that the aim of the cycle of presentations is to show society the effects and repercussions of research conducted in São Paulo State.

“The taxes paid by the people of São Paulo support the research that FAPESP funds,” he said. “It’s important for everyone to know how that research contributes to the development of the state.”

FAPESP CEO Carlos Américo Pacheco recalled that the first meeting in the cycle focused on the ventures supported by FAPESP’s Innovative Research in Small Business Program (PIPE). He added that a third edition of the event would be held on December 11, this time focusing on global climate change.

“As we know, these issues are of great interest to the Legislative Assembly and the general public in São Paulo, and they reflect part of the research agenda funded by FAPESP in recent years. We expect to continue the cycle next year in partnership with ILP,” Pacheco said.