Your Eyes Might Be Full of Bacteria. Here’s Why That’s a Good Thing

The human body is teeming with bacteria, and a lot of it is good for you. Scientists have shown that the microbiome—a community of bacteria that live in the gut and on other places like skin—can protect people against other disease-carrying bugs.

Now, a new study published by researchers at the National Eye Institute (NEI) suggests that the eye—previously thought to be barren of microbes—hosts a community of bacteria that protects against disease. “It has been a controversy,” says study author Rachel Caspi, a senior investigator in the NEI’s laboratory of immunology. “We have proven something that has been a question mark for a long time.”

To reach this discovery, the researchers swabbed the eyelids of mice and discovered the presence of various bacteria, including a type called Corynebacterium mastitidis (C. mast). This bacteria was shown to elicit an immune system response that prevents the growth of pathogens in the eye and keeps microbes at bay.

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But was C. mast actually living on the eye, or did it just happen to be there along with other bacteria? To answer that question, the researchers put two groups of mice into the same living space. One group of mice had C. mast and one group of mice did not. The C. mast bacteria did not spread from one group to the other, indicating that it was actually living in the eye. “It turns out the bacteria doesn’t spread through casual contact but must be passed on from the mother,” says Caspi.

Since the eye is so efficient at getting rid of unhealthy bugs, scientists had previously thought it wasn’t possible for microbes to live there. But the early research suggests otherwise. “Our work shows a proof of principle: that at least one strain of bacteria lives longterm on the surface of the eye, and that it performs a useful function,” says Caspi.

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Whether human eyes contain similar bacterial communities is unknown, but Caspi says it’s likely. “There is no reason why this would be unique to mice,” she says. “The physiology of the ocular surface is similar; it is very likely that what we find in mouse eyes also applies to the human eyes.”

The study also found that when treated with antibiotics, the bacteria in the eyes of the mice became weaker—an argument for more judicious use of antibiotics for eye conditions, Caspi says. A June study found that nearly six in 10 people diagnosed with pink eye in the United States are prescribed antibiotic eye drops, despite the fact that the drugs are rarely needed to treat it.

“We need to be careful with how we use antibiotics, because the penalty is that we eliminate the good bacteria,” says Caspi. “In the future, probiotic therapies could be developed for the eye to make less the use of antibiotics.”