People who wear contact lenses often acquire unwelcome microbial guests along with the convenience afforded by this eyewear. In fact, a higher diversity of bacteria lives on the eye surface of lens wearers than that of the naked-eye crowd, according to an extensive classification effort by microbiologists at New York University's Langone Medical Center. This difference may help explain why lens poppers develop eye infections up to seven times more frequently than they would otherwise.

In an effort to map the ocular microbiome, the researchers sequenced hundreds of swabs from the eyes and eyelids of 11 people who do not wear lenses and nine of those who do. Wearers had about three times the typical proportion of Methylobacterium, Lactobacillus, Acinetobacter and Pseudomonas bacteria. Although the first three bugs are typically harmless, Pseudomonas entering a scratched cornea can result in an infection, triggering redness, pain and blurred vision. Left untreated, the condition may lead to blindness. These same bacterial groups tend to hang out innocuously on our skin, says N.Y.U.'s Lisa Park. This means they most likely hitch a ride on users' fingers during the act of inserting lenses, suggesting an instantaneous shift in the regional microbiome.

Additional results from the study support such a conclusion: the researchers found that the composition of bacteria living on the eyes of people who sport disposable lenses was more similar to that of their skin than was the case among people who don't need lenses. “It's not a definitive connection,” Park says, “but it's very intriguing.” The physical characteristics of the lenses themselves, such as the pressure they place on the eye, could also foster bacterial growth.

All told, the researchers identified about 10,000 distinct strains of bacteria in their samples. Knowing the exact microbial community in a patient's eye could help doctors treat infections with targeted antibiotics, says Jack Gilbert, a microbiologist at the University of Chicago who was not affiliated with the study.

To avoid infections altogether, however, contact lens wearers should assiduously follow best practices with their eyesight enhancers: wash hands thoroughly before handling lenses, use fresh saline solution to rinse and store them, and replace cases every three months. At least that way, the welcome mat for tiny, menacing orbital guests should shrink.