Contact lenses may increase the risk of eye infections by altering the make-up of bacteria living on the eyeball, research has shown.
Researchers found the lenses appeared to transfer bugs from the skin to the eye, upsetting the bacterial ecosystem and triggering infections and inflammation.
Lead scientist Dr Maria Gloria Dominguez-Bello, from NYU Langone Medical Centre in New York City, said: "Our research clearly shows that putting a foreign object, such as a contact lens, on the eye is not a neutral act.
"These findings should help scientists better understand the long-standing problem of why contact-lens wearers are more prone to eye infections than non-lens wearers."
The researchers took hundreds of swabs of various parts of the eyes of nine contact lens wearers and 11 participants who did not wear contact lenses.
They found that in both groups the eye surface, or conjunctiva, harboured a more diverse range of bacteria than the skin directly beneath the eye.
Three times the usual proportion of three types of bugs, Methylobacterium, Lactobacillus, Acinetobacter and Pseudomonas, were identified on the eyeballs of contact lens wearers.
The conjunctival "microbiome", or microbial ecosystem, of lens wearers turned out to be more like that of the skin than of the eye.
Dr Dominguez-Bello added: "What we hope our future experiments will show is whether these changes in the eye microbiome of lens wearers are due to fingers touching the eye, or from the lens's direct pressure affecting and altering the immune system in the eye and what bacteria are suppressed or are allowed to thrive."
The findings were presented at the annual meeting of the American Society for Microbiology in New Orleans.
Co-investigator Professor Jack Dodick, chair of opthalmology at NYU Langone, said: "There has been an increase in the prevalence of corneal ulcers following the introduction of soft contact lenses in the 1970s. A common pathogen implicated has been Pseudomonas.
"This study suggests that because the offending organisms seem to emanate from the skin, greater attention should be directed to eyelid and hand hygiene to decrease the incidence of this serious occurrence."
Some 5,245 distinct bacterial strains and subtypes were identified in the eyes of lens wearers and 5,592 strains in the eyes of non-lens wearers.
Surprisingly, the research showed that more Staphylococcus bacteria - which are linked to eye infections and are more prominent on the skin - were found in the eyes of non-lens wearers. The scientists cannot yet provide an explanation for this result.
