Bacteria, viruses and fungi that colonize the body — including some that live directly on eye surfaces — can have an important role in eye health.
Some Lactobacillus species are thought to be beneficial to the eye.Credit: SCIMAT/Science Photo Library
Science is full of happy little accidents, as Anthony St Leger and his research team have learnt. They were trying to study the microorganisms that live on the surfaces of the eyes of mice, but were having trouble getting the bacteria to grow in the laboratory. Then, there was a moment of serendipity. “We were culturing bacteria and we forgot a plate in a chamber,” explains St Leger, an ocular immunologist at the University of Pittsburgh in Pennsylvania.
Nature Outlook: The human microbiome
When the team discovered the plate a week later, the researchers found that a bacterium was growing on it. Initially, St Leger thought that the plate had been contaminated. But his team repeated the scenario from scratch, this time allowing the microbes to multiply for a week. Once again, the bacterium of interest — Corynebacterium mastitidis — grew.
The bacterium was simply slow-growing and needed more time to multiply. “I say that my whole research programme hinges on that one forgotten plate,” St Leger jokes.
For many years, the eyes were thought to be sterile. But dogged attempts to grow bacteria from tiny samples from around ocular surfaces — and advances in genetic sequencing that can deliver readouts of microbial DNA or RNA without the need to grow the organisms in dishes first — have shown that the eye has its own microbiome.
On the eye surface, there are only around 6 bacteria for every 100 human cells. By contrast, the human colon has an estimated 39 trillion bacteria — the highest density in the body. Some ophthalmologists call the eye ‘paucibacterial’, meaning little or few bacteria. Even so, microbes could have a marked affect on ocular health.
Some types of bacterium, such as Staphylococcus, Streptococcus, Propionibacterium and Corynebacterium, are emerging as influential members of the ocular microbiome. And considerable effort is being put into understanding how microbes in the gut and other areas of the body, affect ocular health. Together, these insights could pave the way for more effective treatments in a range of conditions, including painful dry eye and disorders that cause blindness, such as glaucoma.
Prime suspects
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