Sweating in the gym: research suggests human sweat kills bacteria

Exercise Equipment: How sweat sweeps away gym germs

Woman in the gymIf you're like most people who work out in the gym, you don't particularly enjoy finding the weight bench you are about to use bathed in sweat from the previous occupant; nor do you like picking up dumbbells with wet grips - or sitting down on humid exercise bike seats. What could be more unpleasant? After all, you reason, isn't the sweat on your favourite leg-extension machine - being nothing less than a random sample of someone else's body fluids - simply a repository for whatever pathogens that individual might be harbouring in his/her body?

Concerns about sweaty equipment stimulate many gym athletes to set up their own fitness centres at home. Others take care never to be without a towel when working out at the gym. But does towelling off a hamstring-curl device really remove the germs along with the fluid? Carrying a spray can of disinfectant around the gym is definitely not cool, so what is the gym athlete - surrounded by a sea of apparently unfriendly sweat - supposed to do? Can you sit on someone else's sweat without worry?

According to Birgit Schittek of the Department of Dermatology at Eberhard-Karls University in Tubingen, Germany, you shouldn't sweat it! She and her colleagues have discovered a substance in human sweat that kills many types of common bacteria and yeasts. The compound is so potent that sweat may reasonably be viewed as an anti-infection agent, rather than a fluid festering with disease.

A substance in human sweat kills many types of common bacteria and yeasts

Schittek discovered sweat's key pathogen-fighting chemical - dermcidin - almost by accident. In her primary work with genes involved in the development of the skin cancer malignant melanoma and a related benign condition, she and her co-workers found a unique strip of genetic material with an unknown function. The gene, called DCD, was not active in a variety of human tissues, including the nervous system, digestive organs and blood, but it was expressed strongly in human skin. More specifically, the gene appeared to be most active in cells making up human sweat glands, which are located in the dermis - the thick layer of living tissue that lies beneath the outer layer, or epidermis(1).

After a bit of scientific sleuthing, Schittek worked out that DCD coded for a protein called dermcidin. She was initially unsure of dermcidin's function, but guessed - quite reasonably - that it might have some anti-microbial properties.

After all, human skin contains a number of defences against microbes. The skin itself, containing layers of cells stacked up on each other, represents a physical barrier against would-be invading pathogens. In addition, human skin contains two types of anti-microbial proteins, the 'cathelicidins' and the 'defensins'. Cathelicidins, which appear to kill bacteria via a unique electrical-charge effect, are produced by skin cells called keratinocyctes and are frequently found in skin wounds and sites of skin inflammation. There are a variety of different defensins, which are produced by skin cells as well as white blood cells; some defensins work against 'gram-negative', potentially disease-causing bacteria like E coli, while others go to war with nasty 'gram-positive' bacteria, such as Staphylococcus aureus, a major cause of skin infections.

To see what dermcidin could do, Schittek mixed it - in concentrations typically observed in sweat - with unpleasant skin pathogens such as E coli, S aureus, Enterococcus faecalis (the name suggests its favourite habitat) and also Candida albicans, which is a yeast rather than a bacterium. During four hours of incubation in a basic sodium-phosphate buffer solution with a pH of 7.4 - the same acid-base balance as the blood - dermcidin killed 100% of the bacteria and was also a potent destroyer of C albicans!

Of course, sweat is actually more acidic than blood, with a pH which can range from 4 to 6.8; sweat also contains varying concentrations of sodium (20-60 mM), chloride (20-80 mM), potassium (10 mM), and magnesium (1 mM). To see if dermcidin was active in sweat, Schittek tested the chemical's activity at a pH of 5.5 over a range of sodium concentrations and found that it remained a tough combatant against germs.

'It's very efficient - a very low concentration kills these four (common) microorganisms,' says Schittek. 'It's the first peptide which is constantly produced by the skin. The other antimicrobial peptides, the defensins, are also produced by skin cells, but only after inflammation(2).

From one perspective, sweating is the first line of defence against infection

Schittek is not yet sure how dermcidin actually works. Many anti-microbial proteins possess a positive charge and kill bacteria by disrupting their membranes electrically, but dermcidin's charge is negative. It's also not clear whether dermcidin production increases during heavy sweating (are wetter gym seats safer?) - or if dermcidin synthesis varies significantly between people. One guess is that individuals who are prone to skin infections have naturally low levels of the key defence chemical.

Researchers believe that dermcidin may be effective against bacteria which have become resistant to conventional antibiotics, and they are also intrigued by the possibility that dermcidin might work in other parts of the body as well as the skin. Eventually we may see dermcidin marketed - in ointment or liquid form - as a topical skin application, a somewhat comforting thought the next time you find sweat (and its accompanying dermcidin) applied copiously to your favourite gym appliance. From one perspective, sweating is the first line of defence against infectious agents, so the 'rude' person who left sweat for you may have actually just been cleaning off the equipment!

Owen Anderson

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