understanding the stench could save lives.
Renate Smallegange is something of a connoisseur of
smelly feet – and she goes to surprising lengths to
study their odours. Sometimes she’ll collect worn nylon
socks that have become imbued with the fragrance. If
that’s not good enough, she asks people to rub their feet
on glass beads and wipe their sweaty skin on the
surface. When she’s being really picky, she’ll trap the
feet in a plastic bag, allowing her to draw up the aroma
in gusts of air.
Of all the jobs in the world, it’s certainly not the most
pleasant, but Smallegange is mostly unperturbed by the
occasional whiff of cheese. “It’s not a big deal,” she
tells me. “Of course some people do smell nicer than
others – from my personal point of view.”
Not everyone finds the stink so discomforting. While a
strong stench may cause Smallegange to politely hold
her nose, however, it happens to be a real turn-on for
the other objects of her study: malaria-carrying
mosquitoes. For this reason, Smallegange has been
trying to find the unique recipe that gives our feet their
odour, in a quest to help stem the spread of that deadly
disease.
No matter how clean you are, a slight odour is almost
inevitable, given the anatomy of our feet. The average
foot contains 600 sweat glands per square centimetre –
hundreds more than the armpits . They secrete a
nutritious soup of salts, glucose, vitamins, and amino
acids that provides the perfect diet for a colony of
bacteria. In return for the free lunch, the bacteria leave
us with a cocktail of fatty acids that together give rise to
the signature musk.
There are so many bacteria living on our feet that
microbiologists have had a hard time finding exactly
which species cause the stench, and where they live on
the foot. Showing remarkably little vanity, James
Reynolds at Loughborough University and colleagues
recently attempted to answer this question by mapping
out the populations on their own feet .
Five groups stood out: Corynebacteria, Micrococci,
Propionibacteria, Betaproteobacteria and Brevibacteria – but
the biggest offender appeared to be Staphylococci.
Tellingly, it always seemed to coincide with a
particularly potent chemical, called isovaleric acid. “If
you imagine a well-aged stilton – that’s the smell you
get if you open a bottle of the stuff,” Reynolds says. “If
you spill a drop in the lab you’ll smell it all afternoon –
it’s horrible.” What’s more, they were most common on
the sole, rather than the top – with particular high
numbers around the ball of the foot – perhaps
explaining why these are the smelliest areas. The
comparison to cheese is appropriate. Many cheeses
contain a similar mix of volatile chemicals, with
Limburger cheese apparently offering the closest
comparison.
Eventually, these findings may pave the road to a more
fragrant future. “If we know what these compounds are
and the species that produce them, we could make
clothing that absorbs the smell or neutralises it,” says
Reynolds. It could also lead to better deodorants. The
task will be tricky – alongside the smelly bacteria, our
feet harbour some potentially friendly organisms that act
as gatekeepers against infection. But nature may already
have some answers. A recent Japanese study found that
three chemicals commonly found in citrus fruits can
help target the Staphylococcus bacteria without harming
its neighbours.
In some situations, foot odour is much more serious
than slight embarrassment, however – it might be a
matter of life or death. Dutch scientist Bart Knols was
one of the first to notice that certain species of malaria-
carrying mosquitoes are attracted to the smells wafting
from our feet. His work has since inspired many of
Smallegange’s latest studies at Wageningen University in
the Netherlands.
She has found, for instance, that the malaria parasite
seems to alter the mosquitoes’ sense of smell so that
they are even more attracted to smellier feet, swarming
to worn socks like bees to a flower bed. “There is some
evidence that some proteins in the head of the mosquito
change somehow, and they have a role in the olfactory
response of mosquitoes,” says Smallegange. It’s a
devious mechanism that leads the malaria’s host to
home in on potential victims, so that it can continue its
life cycle in a human body. “It’s why the mosquito is
such a good vector of malaria.”
There are many ways this knowledge could help the
fight against malaria. Smallegange has also examined
whether the particular combination of smell-producing
bacteria on feet can alter your chances of being bitten;
as you might expect, those hosting more
Staphylococcus tend to be more attractive. Attempts to
combat those bacteria could therefore offer some
protection against the deadly disease.
Alternatively, the siren call of the odour could be used
to bait mosquito traps. One idea has been to bait traps
with used socks ; they seem to maintain their allure for
at least 8 days after wear. If that seems like a waste of
good underwear, Knols has also found that Limburger
cheese seems to do the trick. Smallegange, meanwhile,
is hoping to bottle the smell – using isovaleric acid and
other components to produce her own, synthetic odour.
(You could call it “eau de pied”.) “The combination is
very important,” she says. “In general, a blend is more
attractive than a single component.” It’s possible that
you will need to use different recipes for different
species of mosquito, though – each may have their own
particular tastes.
It’s unclear exactly how effective these measures will be
for protection. A small trial in Rusinga Island, Kenya is
currently investigating whether the traps can kill or
distract enough mosquitoes to reduce the overall
number of bites and infections. At the very least, they
could be used for an early warning – to detect whether
malaria-carrying mosquitoes are already breeding
nearby.
For most of us, cheesy feet are no more than an
inconvenience which can be temporarily fixed with a
shower. But Smallegange’s pursuits are worth bearing in
mind, whenever you catch a whiff of your under-soles.
With her collection of smelly socks, skin swabs and “eau
de pied”, she is doing a job that few of us could
stomach, all in the hope of saving lives. And that’s a
mission that’s not to be sniffed at.
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