• Microbiome of the Eye

    Written By: Reena Mukamal
    Jan. 29, 2019

    Your eyes have a microbiome, too

    Microbiomes are communities of organisms including bacteria, fungi and viruses that live on and inside our bodies. Some are harmful and can cause infections, but many parts of the body’s microbiome are essential to good health. Sometimes, the microbiome activates the immune system to get rid of dangerous bacteria and prevent disease.

    In addition to the well-known gut microbiome, your mouth, skin and eyes each have a unique microbiome. The ocular microbiome is a relatively new and emerging area of research and it may lead to new approaches for treatment and prevention of certain eye diseases and conditions.

    The ocular microbiome is a relatively small population

    The community of micro-organisms (flora) in the eye are found on the conjunctiva (the clear tissue covering the white part of your eye) and the cornea. Flora found in the eyelid and eyelashes are considered part of the skin microbiome. Compared to other bodily microbiomes, the ocular surface microbiome is sparsely occupied. If the skin is the Los Angeles of microbiomes, the eye is more like Wichita, Kansas, with roughly 1/100th the number of resident micro-organisms.

    At first scientists believed there were many more bacteria living in the eye, but today they have confirmed that the core ocular surface microbiome for most people has just four species: Staphylococcus, Streptococcus, Propionibacterium and Corynebacterium. This bacterial population is probably so small because your tears are somewhat antimicrobial. Enzymes in tears break down bacterial cell walls and keep them from reproducing.

    Microbiome imbalances may increase risk of eye diseases

    In other microbiomes in the body, imbalances of the native species of bacteria have been shown to affect health and kickstart disease. In the gut, certain forms of colitis are caused by clostridium difficile (C. diff), which can grow unchecked when there is an imbalance of native, healthy bacteria. Researchers hypothesize that the ocular biome could similarly influence risk for conditions such as dry eye disease and endophthalmitis (a severe eye inflammation often caused by infection after eye surgery).

    The ocular microbiome is also home to native viruses

    In addition to bacteria, the healthy ocular surface frequently hosts some viruses, such as the torque teno virus (TTV). The TTV virus has been found in many cases of endophthalmitis but how it gets inside the eye to cause this condition is not yet clear. Other viruses that are part of healthy ocular microbiomes include the Merkel Cell Polyomavirus (MCP) and human papillomavirus (HPV). These viruses, which are usually undesirable, may serve as watchdogs in the ocular microbiome, alerting the immune system when other viruses pop up.

    What microbiome research is being done and how will it influence eye health?

    In the future, researchers plan to investigate the potential connection between the ocular microbiome and eye conditions that damage the ocular surface. These include chronic dry eye and blepharitis (bacteria and oily flakes at the base of the eyelashes).

    Other areas of upcoming investigation include:

    Today, we are just at the beginning of understanding what makes up a healthy ocular microbiome. There is much more to discover.