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When the Bugs Get in Your Eyes

Posted by Ilena Di Toro | Posted on September 26, 2017

You don’t have be an OD or MD to know that having something in your eye is annoying. It can also be harmful to your vision. Irritants and bacteria can eat away at the eye, potentially leading to vision loss.

With that said, there are times when bugs (microbes) can be beneficial to the eye. A study conducted at the National Eye Institute (NEI) and published in the journal Immunity shows the existence of a bacterium that lives on the ocular surface of laboratory mice. This helpful bug actually trains the immune system to fight pathogens.

That’s right, there’s a bacterium living on the surface of mice’s eyes, despite the belief that the surface of the eye was sterile. The laboratory of Rachel Caspi, Ph.D., senior investigator in NEI’s Laboratory of Immunology was able to culture bacteria from the conjunctiva of these mice. The two type of bacteria found are Staphylococci and Corynebacterium mastitidis, also known as C. mast.

Researchers in Caspi’s lab found that when C. mast was cultured with conjunctive immune cells, it lead to the production of interleukin (IL)-17, a protein used for host defense. They also found that IL-17 was produced by gamma delta T cells, which are found in mucous tissue. IL-17 also attracted other immune cells to the conjunctiva and induced the development of anti-microbial proteins in tears.

To find out if the microbes were contributing to the immune response, two groups of mice were studied, a control group that had the C. mast and another group that was treated with antibiotics to kill all bacteria. Then, both groups were exposed to a fungus: candida albicans. The mice treated with antibiotics weren’t able to get rid of the fungus. The inability to fight the fungus led to an ocular infection. The control group was able to fight off the fungus.

Researchers also noticed that mice from NIH had C. mast on their eyes, but mice from Jackson Laboratory in Bar Harbor, Maine and other vendors did not have the bacterium. To see if C. mast was introduced to the mouse’s eye from its environment or whether it had lived there for a period of time, researchers inoculated the C. mast free mice with the microbe and waited to see if it could be cultured from the animals’ eyes weeks later.

When the mice from the from Jackson Laboratory were inoculated with the C. mast, they produced the gamma delta T cells that released the IL-17. The bacteria could be cultured from their eyes many weeks later. As for other types of bacteria that were inoculated into the Jackson Laboratory mice, they disappeared without inducing immunity and C. mast didn’t spread to other mice after eight weeks of co-housing.

“We still don’t know what enables C. mast to successfully establish itself in the eye, whereas other similar bacteria fail to colonize,” Caspi said.

While the C. mast in mice stimulates an immune response, there are situations in humans where it can lead to sickness. For example, seniors have compromised immune systems and C. mast would grow out of control and lead to disease. It is not clear how this finding in mice translates into human health. Still, researchers are now looking whether other bacteria play a role in regulating eye immunity. That’s something to think about when you are trying to get something out of your eye!


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