Cheese, toes, and mosquitoes
A whiff on the evening train announces that the person opposite you has sweaty feet. In a delicatessen, the same smell can be both wholesome and welcome–an indication that you are approaching the cheese counter. Bart Knols and Ruurd De Jong have another use for the reek that emanates alike from unwashed feet and delectable cheese. The Dutch entomologists think they can exploit it to trap mosquitoes and thus help to combat malaria.
Working at Wageningen Agricultural University, they have been investigating the chemical features of humans that attract vectors of malaria, especially Anopheles gambiae. Carbon dioxide in the breath is one powerful attractant. But there are others. Confronted by what Knols and De Jong call a naked, motionless human host, mosquitoes tend to go for the feet and ankles. Their intense interest in those regions correlates well with particular combinations of skin temperature and density of eccrine sweat glands.
However, the researchers have found that washing the subjects’ feet and ankles with non-perfumed but bactericidal soap diverts the mosquitoes to other parts of the body. This has prompted them to investigate odours, especially those that can emanate strongly from the feet and ankles, which may account for A gambiae’s predilection.
For whatever reason, the people of Holland have always been particularly conscious of the similarity between pedal pongs and the aroma of a fine cheese. Their language even has a word, “Tenenkaas” (literally “toes-cheese”), to describe the effluvia generated by perspiring phalanges. And one of the most powerful foot-like stinks of all is produced by a particular pride of the Netherlands–Limburger cheese.
The origin of that characteristic smell is microbial. Limburger is ripened by coryneform bacteria such as Brevibacterium linens, close relatives of which form part of the normal bacterial flora of the feet. The cheese contains short-chain fatty acids, which also occur in human sweat. And among the substances produced by coryneform bacteria in cheese and on sweaty feet is methanethiol, a peculiarly pungent molecule that contributes powerfully to the stink of both.
Maybe Limburger cheese could be used as a bait to attract and trap A gambiae–especially the blood-seeking females that transmit malarial parasites to humans. As they report in Parasitology Today (1996;12:159), Knols and De Jong have now tested the idea. They constructed traps, with or without air blown over Limburger, and exposed them to hungry mosquitoes. In the same period of time, the smelly traps caught more than twice as many A gambiae as those without the smell.
Knols and De Jong caution against too ready an acceptance of the idea that the cheesy odour worked because of its similarity to sweaty feet. But this does seem to be the most likely explanation of their results. As they observe, it is remarkable that mosquitoes so strongly drawn to the human body are also attracted towards an odour from something distinctly non-human.
Unless the two pongs had a common origin. Could some of the bacteria in cheese have come, long ago, from its early makers, sweating over their work? And is today’s concern for scrupulous cleanliness in the dairy depriving future gourmets of delicious aromas still confined to the crevices of the human body?–BERNARD DIXON, European contributing editor, Biotechnology