Friday, December 30, 2011

Skin bacteria affect how attractive we smell to malarial mosquitoes

You could be sitting alone and still be completely outnumbered for your body is home to trillions upon trillions of tiny passengers – bacteria. Your body is made up of around ten trillion cells, but you harbour a hundred trillion bacteria. For every gene in your genome, there are 100 bacterial ones. This is your ‘microbiome’ and it has a huge impact on your health, your ability to digest food and more. We, in turn, affect them. Everything from the food we eat to the way we’re born influences the species of bacteria that take up residence in our bodies. 

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.

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. We are really more like a super-organism consisting of hundreds of species...