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ANAHEIM, Calif. -- Resistance to some antibiotics used to treat ophthalmic infections appears actually to have declined over time, a 7-year study found, although the investigators warned that the problem isn't going away.

Vancomycin, the "gold standard," appeared least vulnerable to resistance, according to Ron Melton, OD, of Charlotte, N.C., who presented the findings here at the .

He also noted that besifloxacin also scored well and is a more accessible alternative since, unlike vancomycin, it doesn't need to be compounded.

On the other side was azithromycin, to which resistance became more common in the study.

The findings came from a 7-year study of 4,226 ocular isolates from clinical centers across the U.S., including eye centers, university hospitals and community hospitals. From 2009-2015, the clinics submitted isolates of Staphylococcus aureus, coagulase-negative staphylococci, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Haemophilus influenzae.

"This is the only ongoing nationwide antibiotic resistance surveillance program focused on common ocular pathogens," said study co-author Randall Thomas, OD, MPH, of Concord, N.C.

Five-year findings from the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) Surveillance Study, funded by Bausch + Lomb, were reported a year ago in JAMA Ophthalmology.

At that time, researchers reported that "methicillin resistance was prevalent among staphylococcal isolates from ocular infections, with many strains demonstrating multidrug resistance." Researchers also said overall ocular resistance did not increase over the 5 years.

In the new study, the researchers report that resistance dropped significantly among S. aureus isolates to oxacillin/methicillin and azithromycin (P≤0.001), among both S. aureus and P. aeruginosa isolates to ciprofloxacin and tobramycin (P≤0.019), and among coagulase-negative staphylococci isolates to ciprofloxacin (P≤0.006).

S. pneumoniae isolates developed a slightly higher level of resistance to azithromycin (P≤0.025), while isolates of H. influenzae were "generally susceptible" to all antibiotics.

The findings confirm that antibiotic resistance remains an issue, said Melton in an interview with MedPage Today. "You should be tracking the most recent data and modifying your treatment based on what's working better at the time," he said.

In the big picture, "besifloxacin is the pick of the litter based on the current data for the common infections we face," said Melton.

It's unlikely that the pathogens will develop significant resistance to the drug, he said. "It's not going to be used systematically, which will dramatically reduce the resistance developing."

As for the worst-performers, Melton pointed to azithromycin, which "just has a lot of problems with common bacteria that we face."

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