Health

Being caked in germs sounds unpleasant, but "friendly" bacteria living on our skin may have the vital role of keeping in check inflammation triggered by injury and unwanted bacteria.

The discovery extends the list of bacteria that the human body relies on to function. It also suggests that antibacterial hand gels and soaps might exacerbate skin conditions characterised by excessive inflammation.

The most common family of bacteria found on the skin is Staphylococcus, the member of which are harmless, unless they get into wounds. To see if they might actually be useful to humans, Richard Gallo at the University of California, San Diego, and his colleagues added molecules released by Staphylococcus to cells found in human skin.

They found that one molecule, called lipoteichoic acid (LTA), stopped some skin cells from releasing chemicals that trigger inflammation, part of the body's immune response. LTA had a similar effect when added to the skin of live mice.

Gut protection

Gallo says that although inflammation is essential for recovery from injury, the ability to damp it down is key because prolonged inflammation can lead to skin diseases like psoriasis.

His team also notes that LTA's protective role seems limited to the skin's surface: in immune cells taken from deeper layers, it provoked inflammation.

Skin Staphylococci aren't the only bacteria we rely on. In the gut, friendly bacteria control inflammation, while in the mouth they may kill strains that cause decay and halitosis

Gallo speculates that bacteria may be also help regulate inflammation in other areas of the body such as the lining of the lungs and nose, although different species of friendly bacteria may be responsible.

Excessive antimicrobials

As for what the Staphylococci get out of this relationship, Gallo says that many of them are resistant to antimicrobial compounds released by human skin that kill off other microbes. "One of the things they are getting out of it is a protective niche with nutrients," he says.

Gallo also cautions against the over-use of antibacterial gels and soaps. Although these might be helpful in curbing transmission of swine flu, excessive or long-term use might also have some negative consequences.

"Any scratch that you might encounter is going to initiate inflammation and the presence of these microbes will help modulate that inflammatory response," says Gallo. "If you do not allow those normal microbes to survive then you lose this ability. That could be something that occurs with excessive use of indiscriminate sterilisers."

Journal reference: Nature Medicine, DOI: 10.1038/nm.2062

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