Bacteria normally found on the skin's surface may play a key role in preventing inflammation and disease.

A new study shows that bacteria living on the skin's surface, including staphylococcal types that typically induce inflammation below the skin, actually prevent excessive inflammation after injury to the skin.

"It provides a molecular basis to understand the 'hygiene hypothesis' and has uncovered elements of the wound repair response that were previously unknown," researcher Richard Gallo, MD, PhD, professor of medicine and pediatrics at the University of California, San Diego, says in a news release. "This may help us devise new therapeutic approaches for inflammatory skin diseases."

The "hygiene hypothesis" emerged in the late 1980s to explain why allergies like hay fever and eczema were less common in children from large families who were exposed to more infectious agents. The theory suggests that a lack of early childhood exposure to infectious agents and microorganisms changes how the immune system reacts to bacterial threats.

In the study, published in Nature Medicine, researchers looked at the role of bacteria found on the surface of the skin in maintaining healthy skin using human and mice cell cultures in the lab.

The results showed activation of a Toll-like receptor 3 (TLR3) was necessary to stimulate normal inflammation after skin injury.

Researchers also found a previously unknown mechanism by which a product of staphylococci bacteria inhibits skin inflammation. The by-product, known as staphylococcal lipoteichoic acid (LTA), acts on the main type of cells found in the outer layer of the skin called keratinocytes via TLR3.

"Keratinocytes require TLR3 to mount a normal inflammatory response to injury, and this response is kept from becoming too aggressive by staphylococcal LTA," says Gallo. "To our knowledge, these findings show for the first time that the skin epithelium requires TLR3 for normal inflammation after wounding and that the microflora helps to modulate this response."

Researchers say the results emphasize the potential benefit of maintaining the balance of bacteria found in healthy skin and the potential negative consequences of altering this balance with the use of topical and systemic antibiotics.