People who wear contact lenses have different microorganisms living on the surface of their eyeballs than those who do not, according to new research.

That may explain why contact wearers tend to have more eye infections, the researchers reported at the annual meeting of the American Society for Microbiology on May 31 in New Orleans.

“As other body sites such as gut, skin and mouth, the eye has a normal community of bacteria, expected to confer resistance to invaders,” senior author Maria G. Dominguez-Bello of New York University School of Medicine said.

“Despite being important in ophthalmology, the eye microbiome has been largely neglected, and its functions remain unknown,” Dominguez-Bello told Reuters Health in a statement.

The researchers swabbed various parts of the eyes, including the eyeball surface and the skin directly beneath the eye, of nine contact lens wearers and 11 non-lens wearers at three different times. They used so-called metagenomic sequencing to identify the specific microorganisms in each swab.

For contact wearers, the bacterial population living on the eyeball surface was more similar to that of the eyelid skin than for non-lens wearers, the researchers found. Contacts may transfer bacteria from the finger skin to the eye surface, they write in their poster presentation.

For both groups, there were more than 5,000 bacterial strains on the eye surface and more than 2,000 strains on the skin directly beneath the eye.

This was unexpected, since tears contain antibacterial compounds, Dominguez-Bello said.

Non-lens wearers tended to have more bacterial strains than lens wearers in each location.

But for lens wearers the proportion of bacteria from the Methylobacterium, Lactobacillus, Acinetobacter and Pseudomonas families was three times as great as for non-lens wearers.

“The findings indicate there are significant differences in diversity of bacterial sequences between contact and non-contact lens wearing subjects and that taxonomic units in the contact lens wearing group were more like the skin than the normal eye,” Dr. Stephen Pflugfelder, who studies cornea and ocular surface diseases at Baylor College of Medicine in Houston, Texas, said.

“Because they did not perform cultures, the study didn’t determine if these sequences were from live viable bacteria or dead bacteria that may have been transmitted by the finger skin,” Pflugfelder, who was not part of the new research, told Reuters Health by email.

Previous studies have shown the eye is very sparsely colonized by microbes compared to other areas of the body, such as the mouth, according to Mark Willcox, the Brien Holden Vision Chair in Experimental Optometry at the University of New South Wales in Sydney, Australia.

“So, it seems unlikely that the ocular microbiota plays a large part in protecting the eye from colonization by potential pathogens,” Willcox, who was not involved in the research, told Reuters Health by email.

Contact lens wearers are at about 80 times greater risk for corneal infection, or keratitis, than those who do not wear lenses, he said.

Pseudomonas, which was three times as common among lens wearers in the new research, is most often responsible for lens wearers’ keratitis, he said.

Disinfecting the lenses when not being worn and washing hands with soap and water and drying them before touching the lenses reduces infection risk, he said.

Further studies will need to determine whether these bacteria are transferred from the fingers to the lens and to the conjunctiva or whether the lenses act to favor skin-like bacteria in the eye microbiota, Dominguez-Bello said.

“It’s too early to give advice yet, but the results start to indicate why contact lens wearers are more prone to have eye infections than non wearers,” she said.