Mosquito-fighting bacteria

A bacterium is a better mosquito-repellent than commercially available products...

Mosquito bites can be more than just itchy reminders of warm get-aways: they can transmit bacterial, viral and parasitic infections. And with over 40% of the world’s population at risk from malaria, and mosquitoes as key vehicles of dengue, chikungunya, West Nile and Zika viruses, better insect repellents have never been more sorely - or itchily - needed.

Currently, the most widely-used insect repellents contain the chemical DEET (N,N-Diethyl-meta-toluamide), which was developed in the1940s and used successfully since. But DEET does have its downsides, including the risk of skin irritation and toxicity in some biological systems. These constraints limit the concentrations that can be used, and therefore the effectiveness of the repellent effect. Consequently, scientists are searching for friendlier and more effective alternatives.

Most of the candidates discovered to date have been based on plant products. But now, in an article published in Science Advances, University of Wisconsin-Madison researcher Mayur Kajla and his colleagues have discovered that chemicals produced by the bacterium Xenorhabdus can deter multiple mosquito species from feeding. And they do it with equivalent - or better - efficacy than DEET.

The insect-repelling activity of the bacterial products was tested in the laboratory by tracking mosquito landing and feeding on a warmed-blood chamber protected by a membrane, both in the presence and absence of the repellent substances. This system has the advantage of requiring low doses of test substance, and does not carry the same risks of contamination or inter-subject variation that accompanies animal or human testing.

The bacterial products in question are compounds known as fabclavines, which have insecticidal and antiobiotic activities. The molecules are secondary metabolites made by the Xenorhabdus. In their natural environment, which is the body of a nematode worm that normally infects insects, the bacteria probably use these chemicals to boost their survival rates. 

They appear to target insect taste or smell pathways, although the exact way in which they do this, and hence work as mosquito repellents, is not yet.

But important considerations remain to be investigated before the discovery can be translated beyond the laboratory. These include safety, production cost-effectiveness and mode of administration tests: will direct application onto the skin work, for example; or would the best approach be indoor spraying and insecticide-treated nets?

These bacterial products represent an exciting new avenue for public health measures, illustrating how an improved characterization of bacterial genomes and activities can lead to real-world impact!