Powerful Mosquito-Repelling Compounds Isolated from Rare Bacterium

A mixture of mosquito-repelling compounds isolated from a Gram-negative bacterium called Xenorhabdus budapestensis exhibits potent feeding-deterrent activity against three important mosquito species (Aedes aegypti, Anopheles gambiae, and Culex pipiens), and works at lower doses than repellents currently on the market, including DEET and picaridin, according to a study published in the journal Science Advances.

“Whether these natural chemical compounds, called fabclavines, are suitable for human use remains to be determined, but the study opens up a new area of exploration in the search for insect-repelling and insect-killing compounds,” said study senior author Professor Susan Paskewitz, a researcher in the Department of Entomology at the University of Wisconsin-Madison.

“We didn’t come at it thinking we would find a repellent. It was a bit of serendipity.”

Before the study, Professor Paskewitz and colleagues already knew that extracts from Xenorhabdus budapestensis, a bacterial symbiont of Steinernema bicornutum nematodes, did not kill mosquitoes but when the extracts were added to their food, the mosquitoes refused to eat.

They designed a set of experiments to test the repellent potential of the bacterial extract and identify the compounds responsible.

Using a commercial mosquito feeding system, they made modifications to more closely mimic a mosquito feeding on a human being. For instance, they selected a skin-like membrane to contain a special, red-dyed mosquito diet that simulates human or animal blood. They also tested a variety of cloth coverings to sit atop the membrane, which would be coated with the repellents being screened.

The researchers coated the cloth with water, DEET or picaridin and allowed mosquitoes to feed for 30 minutes before freezing them and counting the number that were engorged with red liquid (fed) or unfed. The mosquitoes did not feed when the cloth was coated in repellent.

They then tested purified extracts from the bacteria and found that an extract dominated by two fabclavine molecules effectively deterred mosquitoes from feeding.

When compared for effectiveness against picaridin and DEET, which is found in more than 500 insect repellents registered with the U.S. Environmental Protection Agency, the bacterial extract was effective at doses eight times and three times lower than each, respectively.

“If you can use less of an active ingredient in a formulation it may be less expensive,” Professor Paskewitz said.

“The compounds extracted from Xenorhabdus budapestensis may yet prove useful for other applications and continues to explore their potential,” said study first author Dr. Mayur Kajla, also from the Department of Entomology at the University of Wisconsin-Madison.

“These compounds might end up being more effective against a wider array of biting arthropods,” Professor Paskewitz added.

“DEET works against ticks, but it’s not as good as it is with mosquitoes. We will test the bacterial compounds against other kinds of biting insects and their relatives.”

_____

Mayur K. Kajla et al. 2019. Bacteria: A novel source for potent mosquito feeding-deterrents. Science Advances 5 (1); doi: 10.1126/sciadv.aau6141