Your skin is teeming with bacteria. There are billions of them, living on the dry parched landscapes of your forearms, and the wet, humid forests of your nose. On your feet alone, every square centimetre has around half a million bacteria. These microbes are more than just passengers, hitching a ride on your bodies. They also affect how you smell.
Skin bacteria are our own natural perfumers. They convert chemicals on our skin into those that can easily rise into the air, and different species produce different scents. Without these microbes, we wouldn’t be able to smell each other’s sweat at all. But we’re not the only ones who can sniff these bacterial chemicals. Mosquitoes can too. Niels Verhulst from Wageningen University and Research Centre has just found that the bacteria on our skin can affect our odds of being bitten by a malarial mosquito.
We’ve all heard about “beer goggles”, the mythical, invisible eyewear that makes everyone else seem incredibly attractive after a few pints too many. If only beer had the reverse effect, making the drinker seem irresistibly attractive. Well, the good news is that beer does actually do this. The bad news is that the ones who are attracted are malarial mosquitoes.
Anopheles gambiae (the mosquito that transmits malaria) tracks its victims by their smells. By wafting the aromas of humans over thousands of mosquitoes, Thierry Lefevre found that they find the body odour of beer drinkers to be quite tantalising. The smell of tee-total water drinkers just can’t compare. The somewhat quirky conclusion from the study, albeit one with public health implications, is that drinking beer could increase the risk of contracting malaria.
Lefevre recruited 43 men from Burkina Faso and sent them individually into one of two sealed, outdoors tents. One tent was kept unoccupied. In the second, the volunteer had to drink either a litre of water (just shy of two pints) or a litre of dolo (a local 3% beer and the country’s most popular alcoholic drink). A fan pumped air from the tents, body odour and all, into the two forks of a Y-shaped apparatus. Both branches met in a third arm, which ended in a cup full of mosquitoes. The insects had to decide which branch of the Y to fly down and two pieces of gauze trapped them in their chosen path (and saved the volunteers from an infectious bite).
Lefevre showed that the smell of a beer drinker, 15 minutes after chugging his litre, increased the proportion of mosquitoes inclined to fly into the tubes, and the proportion (65%) who headed down the beer-scented fork. The smell of water-drinkers had no effect, nor did the smell of the occupied tent before its inhabitant started drinking.
This is an updated version of the first post I wrote this year. The scientists in question were looking at ways of recruiting bacteria in the fight against mosquito-borne diseases, such as dengue fever. They’ve just published new results that expand on their earlier experiments.
Mosquitoes are incredibly successful parasites and cause millions of human deaths every year through the infections they spread. But they are no match for the most successful parasite of all – a bacterium called Wolbachia. It infects around 60% of the world’s insect species and it could be our newest recruit in the fight against malaria, dengue fever and other mosquito-borne infections.
Wolbachia doesn’t usually infect mosquitoes but Scott O’Neill from the University of Queensland is leading a team of researchers who are trying to enlist it. Earlier this year, they published the story of their first success. They had developed a strain that not only infects mozzies, but halves the lifespans of infected females. Now, as the year comes to an end, they’re back with another piece of good news – their life-shortening bacteria also guard the mosquitoes from other infections.
It protects them against a species of Plasmodium, related to the parasite that causes malaria in humans, as well as the viruses responsible for dengue fever and Chikungunya. Infected insects are less likely to carry parasites that cause human disease, and those that do won’t live long enough to spread them. It’s a significant double-whammy that could have a lot of potential in controlling mosquito-borne diseases.
To our ears, the buzz of a mosquito is intensely irritating and a sign of itchiness to come, but to theirs, it’s a lover’s serenade. The high-pitched drone of a female is a siren’s song that attracts male mosquitoes. And a new study shows that when the two love-bugs meet, they perform a duet, matching each other’s buzzing frequency with careful precision.
The female Aedes aegypti mosquito (the carrier of both dengue and yellow fever) beats her wings with a fundamental frequency of about 400Hz, producing a pitch just slightly lower than concert A. Males on the other hand, have a fundamental frequency of around 600Hz, about one D above middle C.
Lauren Cator and colleagues from Cornell University discovered the sonic secrets of courting mosquitoes by tethering individuals to pins and moving the females past the males. On two-thirds of these fly-bys, the amorous mosquitoes harmonised. Neither took the lead – instead, both buzzers shifted their flight tones so that the male’s second harmonic (the second multiple of his fundamental frequency) and the female’s third had a mutual frequency of about 1,200 Hz. They synchronised in this way for about 10 seconds.