December 2016




Presentation spotlight

Eyeing ocular antibiotic resistance trends

by Vanessa Caceres EyeWorld Contributing Writer

Ocular infection

Antimicrobial resistance among staphylococci remains high but may be decreasing; resistance among S. pneumoniae to azithromycin doubles

When treating a patient for an ocular infection, there’s no antibiotic that can effectively treat all infections without any resistance issues, said Penny Asbell, MD, professor of ophthalmology, Icahn School of Medicine at Mount Sinai, New York.

That’s the take-home message from Dr. Asbell and fellow researchers’ work with the Ocular TRUST and ARMOR studies, both of which have focused on antibiotic resistance. Dr. Asbell spoke in detail about Ocular TRUST and ARMOR during a presentation at the 2016 American Academy of Ophthalmology (AAO) annual meeting in Chicago.

Results from the ARMOR study—short for Antibiotic Resistance Monitoring in Ocular Microorganisms—have been published previously, and research is ongoing.1 The results published from ARMOR involved isolates collected from clinical centers around the U.S. between 2009 and 2013 and January to May 2015.

The Ocular Tracking Resistance in U.S. Today (TRUST) results were published in 2008 and involved ocular isolates that were prospectively collected.2 A look at the results from both studies gives clinicians and researchers a big picture view of ocular antibiotic resistance trends, Dr. Asbell said. Although there were many potential isolates that the researchers could have selected, they decided to focus mostly on gram-positive organisms, Dr. Asbell said. Researchers also focused on Staphylococcus resistance because Dr. Asbell’s previous research has found increasing resistance to S. aureus isolates.3

Ocular TRUST and ARMOR both focused on staphylococcal and pneumococcal bacteria and aimed to evaluate resistance trends over time. Data were combined to report resistance rates. The minimum inhibitory concentrations for relevant antibiotic classes were determined by broth microdilution in both studies; susceptible, intermediate, or resistant determination was based on systemic breakpoints.

More than 5,000 isolates have been analyzed, with a little more than 1,000 with Ocular TRUST and more than 4,000 with ARMOR. “One problem with a single site [study] is you would never get these kinds of numbers, especially for ocular isolates,” Dr. Asbell said.

Of the 1,900+ S. aureus isolates, 38% were methicillin-resistant S. aureus (MRSA). The antibiotic trimethoprim did well in the analysis, but all of the antibiotics that were analyzed had some resistant organisms. “It’s hard to find one where you’re 100% home free,” she said. The antibiotics analyzed were azithromycin, oxacillin/methicillin, ciprofloxacin, levofloxacin, gatifloxacin, moxifloxacin, tobramycin, and trimethoprim. When researchers examined S. aureus resistance by year (2006 to 2015), they found a small but significant drop in resistance against oxacillin, ciprofloxacin, and tobramycin. The clinical significance, however, is not clear. “Whether this is an ongoing trend, we’ll only be able to tell as we go forward,” she said.

Among coagulase-negative staphylococci (1,456 isolates), 50% were methicillin-resistant. “Azithromycin doesn’t come out too well,” Dr. Asbell said. “On the other hand, we get to trimethoprim and there is also resistance there. Again, we’re not finding one [antibiotic] that is home free.” The year-by-year analysis (2007 to 2015) found a small but significant decrease in resistance against oxacillin and ciprofloxacin. However, the clinical significance of these changes is not known; further surveillance studies may help determine trends going forward.

Of the S. pneumoniae isolates (789 isolates), 34% were azithromycin resistant. That analysis included azithromycin, levofloxacin, gatifloxacin, and moxifloxacin. A year-by-year examination (2008 to 2015) revealed a significant increase in resistance against azithromycin.

In a potency analysis that was part of the research, vancomycin and besifloxacin performed well, Dr. Asbell said. The study also revealed a dip in methicillin-resistant S. aureus in 2014 and 2015, Dr. Asbell said. “Maybe we’re doing better systemically and topically with antibiotic use,” she said. However, there has been an increase in recent years in methicillin-susceptible strains. Researchers will need to perform continued surveillance to see how these trends bear out over time, and clinicians can use the surveillance data to help make better treatment options, Dr. Asbell said.

Bausch + Lomb (Bridgewater, New Jersey) began the ARMOR studies in 2009 to analyze antibiotic resistance during the treatment of bacterial infections. It is the only surveillance program of its kind in the U.S. that specifically monitors ocular pathogens, Dr. Asbell said.


1. 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;133:1445–1454.

2. Asbell PA, et al. Ocular TRUST: nationwide antimicrobial susceptibility patterns in ocular isolates. Am J Ophthalmol. 2008;145:951–958.

3. Asbell PA, et al. Increasing prevalence of methicillin resistance in serious ocular infections caused by Staphylococcus aureus in the United States: 2000 to 2005. J Cataract Refract Surg. 2008;34:814–818.

Editors’ note: Dr. Asbell has financial interests with Bausch + Lomb.

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