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.
The mosquito that carries malaria – Anopheles gambiae – tracks its victims with an acute sense of smell. It can track the plumes of carbon dioxide that we exhale, and it’s also attracted to the mix of smelly chemicals given off by our skin. We know that some people smell much more attractive to mosquitoes than others. Our diet matters – for example, drinking beer can give our body odour an irresistible allure.
From past experiments, we know that our skin bacteria are also important. Human sweat becomes more enticing to A.gambiae after it is incubated with skin bacteria for a few days. Even on their own, the bacteria can produce airborne chemicals that attract mosquitoes.
These studies were done using bacteria cultured in a laboratory; Verhulst wanted to analyse the microbes on real skin. He rubbed glass beads against the feet of 48 volunteers to sample their scents and offered the beads to captive mosquitoes. Nine of the volunteers proved to be mosquito magnets, while seven others were almost invisible to the blood-suckers.
These two groups had important differences in their skin bacteria. The attractive ones had more bacteria on their feet than the unappealing ones, but they also had a narrower diversity of microbes; their communities were larger, but more gentrified.
Verhulst even managed to identify specific bacteria that drew in or put off the mosquitoes. People with lots of Staphylococcus or Variovorax were more attractive, while those rich in Pseudomonas, Leptotrichia, Delftia and Actinobacteria were not.
It’s not hard to see how these studies could eventually help us to develop new chemicals that attract mosquitoes into traps, or repulse them from humans. But there’s a lot of work to do first. For Verhulst and his team, the most obvious next move is to work out which chemicals the different skin bacteria are giving off.
For example, what is it about the presence of Pseudomonas that shields us from the attention of mosquitoes? Do they produce chemicals that actively repel the insects? Do they break down the attractive compounds of other bacteria or mask their effects? Do they crowd out bacteria like Staphylococcus, which produce more appetising scents? And why do people with more diverse skin bacteria smell less inviting? Perhaps among a larger multitude of species, it’s more likely that one of them secretes chemicals that drive mosquitoes away, or interferes with their sense of smell.
Finally, are people with certain skin bacteria actually more likely to contract malaria? And would it ever be possible to reduce the risk of malaria, or other infectious diseases, by actively changing the communities on our skin to alter the way we smell?
Verhulst’s study demonstrates, yet again, that we are more than just a collection of human cells. We’re also the sum of the bacteria and other microbes that live within us. These legions affect the way we smell, and thus our risk of disease. They, in turn, are affected by us: immune system genes can influence our body odour, perhaps by dictating which bacteria can set up shop on our skin. It is a convenient fiction to talk of ourselves as single creatures; we are really more like a super-organism consisting of hundreds of species, only one of which is human.
Reference: Verhulst, Qiu, Beijleveld, Maliepaard, Knight, Schulz, Berg-Lyons, Lauber, Verduijn, Haasnoot, Mumm, Bouwmeester, Claas, Dicke, van Loon, Takken, Knight & Smallegange. 2011. Composition of Human Skin Microbiota Affects Attractiveness to Malaria Mosquitoes. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0028991