The ecosystem on your forearm

"The number of human beings who have ever lived on earth is dwarfed by the number of bacteria resident in your colon."

A whole new world. Photo: Getty Images

In certain sections of the media, ’tis the season to be talking about bikini bodies. That basically translates to finding the Holy Grail of near-instantaneous weight loss. Why fight the trend?

Here’s how to dump a kilo from your body weight – get rid of the bacteria colonising your body. Ninety-five per cent of the biological cells you carry around with you are not your own; they belong to bacterial organisms that see you as a useful host. To be slightly distasteful for a moment, here are a couple of fun facts: kissing your partner (or anyone, for that matter) creates a suction vortex that rips bacteria from their teeth and pulls them into your mouth. Not gross enough for you? Try this: the number of human beings who have ever lived on earth is dwarfed by the number of bacteria resident in your colon. And this colonic colony is a movable feast; you defecate your own weight in bacteria every year.

Don’t be alarmed. This is a prime example of symbiosis, creatures living happily together side by side for mutual benefit. Stripping away your gut bacteria would be a terrible idea. They are an integral part of your digestive processes. They also work in tandem with your immune system, sounding the alarm and rallying to the fight when something nasty comes along. Most of your bacterial invaders are looking out for you.

The big surprise, published recently in the journal Science, is that your skin is also covered by a network of bacteria that help keep you safe from harm.

Take a close look at the back of your hand and you will see a diverse set of natural habitats. Bacteria are living in the wrinkles of your knuckles and the pools of natural oils around each hair follicle. If you had a microscope handy, you might just be able to find creatures such as Propionibacterium acnes and Staphylococcus aureus. Less visible parts of your outer layer play host to other organisms; on your skin, there are about 1,000 species of bacteria. Your forearm has the biggest biodiversity. It probably hosts around 40 different species.

Human skin is just under two metres squared of prime real estate but the bacteria don’t live there rent-free. Research carried out at the US National Institute of Allergy and Infectious Diseases has shown that if you strip the skin of all bacteria, it compromises the immune system. Introduce a potentially harmful bacterium to this sterilised skin and the skin’s infection-fighting T cells respond in a lacklustre manner. Putting a common human skin bacterium, Staphylococcus epidermidis, back on to the skin fully restores the T cell response to threats.

The researchers revealed the full glory of this symbiotic relationship when they repeated the experiment but chemically silenced the immune system so that it couldn’t exchange messages with the natural skin bacteria. The immune response to harmful bacteria crumbled away once more. The communication between the bacteria that cover your body and your immune cells appears to be what makes you so good at fighting off infections.

Strange alliance

If the thought of all those microbes living on your skin makes you itch, just imagine how itchy you might be without them. The competition for skin space between the various bacteria keeps all the populations in check. Rhinos have oxpecker birds that keep parasites at bay, moray eels have their cleaner shrimps, and you have Staphylococcus epidermidis.

So, perhaps it is worth noting that many of your skin bacteria are sensitive to ultraviolet radiation. Should you expose that bikini body to UV rays, bear in mind that you might be killing off some of your greatest allies.

Michael Brooks’s “The Secret Anarchy of Science” is published by Profile Books (£8.99)