You may be familiar with the idea that your gut and skin are home to a collection of microbes—fungi, bacteria and viruses—that are vital for keeping you healthy. But did you know that your eyes also host a unique menagerie of microbes? Together, they’re called the eye microbiome. When these microbes are out of balance—too many or too few of certain types—eye diseases may emerge.

I’m an immunologist studying how the eye prevents infection. I became interested in this field because humans get only two eyes, and understanding how bacteria affect immunity may be the key to avoiding up to 1 million visits to the doctor for eye infections and save US$174 million per year in the U.S. alone.

When discussing the microbiome, most scientists usually think of the gut, and deservedly so; researchers think one colon can harbor more than 10 trillion bacteria. That being said, more attention is now being focused on the impact microbiomes have at other sites, including the skin, and areas with very few bacteria, like the lungs, vaginaand eyes.

For the last decade, the role of the microbiome in ocular health was controversial. Scientists believed that healthy eyes lacked an organized microbiome. Studies showed that bacteria from the air, hands or eyelid margins could be present on the eye; however, many believed these microbes were simply killed or washed away by the continual flow of tears.

Only recently have scientists concluded that the eye does, indeed, harbor a “core” microbiome that appears dependent on age, geographic region, ethnicity, contact lens wear and state of disease. The “core” is limited to four genera of bacteria Staphylococci, Diphtheroids, Propionibacteriaand Streptococci. In addition to these bacteria, torque teno virus, implicated in some intraocular diseases, also counts as a member of the core microbiome as it is present on the surface of the eye of 65% of healthy individuals.

This suggests that doctors should think more deeply about the risks and benefits to the microbiome when prescribing antibiotics. The antibiotics may kill bacteria that are providing a benefit to the eye.

In a recent study spanning more than a decade and including more than over 340,000 patients in the U.S., the authors found that antibiotics were used to treat 60% of acute conjunctivitis (pink eye) cases. But viral infections are the most likely causes of pink eye, and not treatable with antibiotics. More striking, even cases caused by bacteria often resolve in 7-10 days without intervention. It is well known that excessive or inappropriate antibiotic use can disrupt the microbiome, leading to infection, autoimmunity and even cancer.

Within the past decade, studies assessing the eye microbiome and disease have boomed. They’ve generated an immense amount of data, but most of it is correlative. This means that certain bacteria have been linked to certain diseases, like Sjogren’s Syndrome or bacterial keratitis. However, whether these bacteria are causing these diseases is still unknown.

In 2016, ocular immunologist Rachel Caspi at the National Eye Institute and I hypothesized that protective bacteria were living near or on the eye. Indeed, we found a resident bacterium, Corynebacterium mastitidis (C. mast), that stimulates immune cells to produce and release antimicrobial factors that kill harmful microbes into the tears.

Through a series of experiments, the Caspi lab was able to show for the first time a causal relationship between C. mast and a protective immune response. Whenever C. mast was present on the eye surface, mice were more resistant to two species of bacteria known to cause blindness: Candida albicans and Pseudomonas aeuruginosa.

Now, in my lab, we would like to exploit this relationship between C. mastand ocular immunity to develop novel therapies to prevent infection and possibly target more widespread diseases like Dry Eye Disease.