WASHINGTON, July 11 (Xinhua) -- The surface of the eye is not sterile as previously thought, a new mice study said Tuesday.

Some microbes can live on this sparsely populated tissue and may play a key role in fending off pathogens to prevent eye infection, according to the study published by the U.S. journal Immunity.

"People have been finding bacterial DNA on the human eye but no one has presented experimental proof that these bacteria actually live there," said senior co-author Rachel Caspi, an immunologist at the U.S. National Eye Institute, part of the National Institutes of Health (NIH).

"For all we know, these bacteria may have 'crash landed' on the eye and were killed by the antimicrobials in tears or patrolling immune cells. Finding a bug that persists there long term was a bit of a surprise. We were looking for it but not necessarily expecting it."

In the study, the researchers swabbed samples from the mouse conjunctiva, the membrane that lines the eyelids, and discovered the existence of some some microbes, including one called Corynebacterium mastitidis, or C. mast.

They found that mice with C. mast could fend off invading pathogenic microbes better than mice without it.

Further experiments established that C. mast could induce the production of an immune molecule called IL-17, which summons immune cells to areas of infection to kill invading pathogenic microbes.

These results challenge the notion that the surface of the eye doesn't have any full-time microbial residents, the researchers said.

"We know that C. mast must be a 'permanent resident' as opposed to a 'guest' because it has to be instilled onto the eye, or acquired from the mother in infancy. It does not transfer from one adult mouse to another in the same cage, even after weeks of co-housing," said first author and NIH postdoc Anthony St. Leger.

Although C. mast appears to stimulate a beneficial immune response, there may be situations in which it could cause disease, St. Leger noted.

For instance, the elderly tend to have suppressed immune systems, which might allow C. mast to grow out of control and cause disease.

The researchers are currently investigating whether other bacteria play a role in regulating eye immunity.

"We don't think that C. mast is the only commensal. This is a proof of concept," said Caspi. "There's no question in our minds that other commensals will also be found at the ocular surface."