August 2019


Ongoing study tracks antibiotic resistance of ocular pathogens

by Liz Hillman EyeWorld Senior Staff Writer

A laboratory petri dish with Staph colonies
Source: Penny Asbell, MD

The only ongoing, annual, nationwide surveillance study of antibiotic resistance in ocular pathogens in the United States has released preliminary information from its latest set of findings, showing some changes in resistance, both positive and negative.
The Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) study, funded by Bausch + Lomb, began in 2009. As part of the study, researchers have been conducting yearly tracking and antibiotic susceptibility testing of ocular infections resulting from Staphylococcus aureus, coagulase-negative staphylococci (CoNS), Streptococcus pneumoniae, Pseudomonas aeruginosa, and Haemophilus influenzae.1,2,3
From 2009 to the preliminary data from 2018, 2,108 S. aureus isolates have been collected. During this timeframe, researchers observed a decrease in methicillin resistance in S. aureus, with 40% of S. aureus isolates in 2018 being methicillin resistant.
Though a decrease in resistant strains is positive, 40% is “still quite high,” said Penny Asbell, MD, coinvestigator.
Many surveillance studies began because of the alarming rate of methicillin resistance observed in staphylococci.
“If you look at the trend at the beginning of this century, it looked like [resistance] was on the rise and it was going to reach 100%. We don’t know what the future will bring, but at the moment it looks like it’s topped off,” Dr. Asbell said.
Dr. Asbell hypothesized this could be due to more awareness about methicillin resistance leading to more judicious use of antibiotics, both among patients and in livestock animals.
“Careful use of antibiotics might lead to less or slower development of resistant organisms. One of the key take-homes in any antibiotics discussion is don’t overuse them,” Dr. Asbell said.
There was a significant decrease in S. aureus resistance to azithromycin, ciprofloxacin, tobramycin, and chloramphenicol as well.
Though S. aureus saw a decreasing trend of methicillin resistance, there was no such decrease among coagulase-negative staphylococci. Since 2009, 1,721 coagulase-negative staphylococci were collected. About half of these exhibited methicillin resistance each year. There was increased resistance to tobramycin and decreased resistance to ciprofloxacin over the 10-year timeframe.
The preliminary findings were presented at the 2019 Association for Research in Vision and Ophthalmology annual meeting. The data indicate that when an organism is methicillin resistant, it is more likely to be resistant to other antibiotics as well.
“Once you identify that a patient isolate is methicillin resistant, that’s when the ophthalmologist or treating clinician should have a heightened level of concern to determine the best options given what we know from surveillance studies and the laboratory report,” Dr. Asbell said.
The findings indicate that antibiotic resistance is extremely common, and thus, every eyecare provider is going to come across patients with infections from resistant organisms, Dr. Asbell said.
“It’s good for all clinicians to understand that resistance is part of the picture when we’re treating ocular infections, whether it’s conjunctivitis, corneal ulcer, or, hopefully rarely, endophthalmitis. With that, we can be more thoughtful and intelligent in how we pick antibiotics,” Dr. Asbell said.
Vancomycin, Dr. Asbell said, continues to work well against many resistant staph organisms. However, it’s not FDA approved nor is it commercially available (it has to be compounded). Besifloxacin, which is commercially available for topical ocular use, also seems to do well at killing 90% of isolates at a low concentration. Fluoroquinolones, which are commonly used, have high rates of resistance to staphylococcal organisms.
“Some of the topical antibiotics we commonly use are described as ‘broad spectrum’; however, broad spectrum does not equate to being 100% effective. They may not work, and if they don’t work, that’s when we should be thinking about resistance,” she said. “Our real go-to for serious gram-positive infections still appears to be vancomycin.”
The ongoing ARMOR study includes 15 sites in the U.S. These sites are invited to submit up to a maximum number of ocular isolates per collection year that are then sent to a central laboratory that conducts standardized antibiotic resistance analysis on all isolates collected each year.

About the doctor

Penny Asbell, MD
Chair, Department of
Barrett G. Haik Endowed Chair
Director, Hamilton Eye Institute
University of Tennessee Health Science Center
Memphis, Tennessee


1. Thomas RK, et al. Antibiotic resistance among ocular pathogens: current trends from the ARMOR surveillance study (2009–2016). Clin Optom. 2019;11:15–26.
2. Asbell PA, Sanfilippo CM. Antibiotic resistance trends among ocular pathogens in the US – Cumulative results from the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) surveillance study. US Ophthalmic Rev. 2017;10:35–8.
3. Asbell PA, et al. Antibiotic resistance among ocular pathogens in the United States: Five-year results from the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) surveillance study. JAMA Ophthalmol. 2015;133:1445–54.

Financial interests

Asbell: None

Contact information


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