ARMOR surveillance study update gives current insights on antibiotic resistance for clinical ocular isolates

Reviewed by Penny Asbell, MD

The most recent analyses of data from the ARMOR (Antibiotic Resistance Monitoring in Ocular micRoorganisms) Surveillance Program can guide clinicians choosing antibiotic therapy for initial empiric therapy and infection prophylaxis. The information also reinforces the importance of prudent antibiotic prescribing to limit the development of bacterial resistance to existing options, according to Penny Asbell, MD.

Analyses of data collected during the first 10 months of 2016 show that the level of antibiotic resistance continues to be high among staphylococcal isolates and especially among strains that are methicillin-resistant (MR). Furthermore, the rate of multidrug resistance for these microorganisms, i.e., resistance to 3 or more classes of antibiotic, remained high, especially among the MR strains.

“Susceptibility data collected during ARMOR provide information on current antibiotic resistance levels and trends over time for common ocular pathogens.  Based on the reported data, ophthalmologists can make informed decisions when selecting effective treatments for their patients,” said Penny Asbell, MD, Professor of Ophthalmology, Icahn School of Medicine at Mount Sinai, and Director, Cornea and Refractive Surgery Center, Mount Sinai Hospital, New York, NY. 

“These most recent data from the ARMOR study also emphasize previous findings that antimicrobial resistance continues to be a serious threat to ocular health, with many strains demonstrating resistance to multiple drug classes, and clinicians are encouraged to exercise judicious antibiotic use.”

The 2016 analysis included 359 isolates that were collected from 11 US centers that participate in ARMOR. The review of the minimum inhibitory concentration (MIC) data showed that all isolates of Haemophilus influenzae were susceptible to all antibiotics tested. Resistance among Pseudomonas aeruginosa isolates to fluoroquinolones was 7%, and although the rate is relatively low, it is more than twofold greater than the resistance rate in 2015.

For Streptococcus pneumoniae, about one-third of isolates exhibited non-susceptibility to azithromycin and to penicillin, but the susceptibility of these Gram-positive organisms to fluoroquinolones and chloramphenicol remained high.

Resistance rates for Staphylococcus aureus and coagulase-negative staphylococci (CoNS) were generally unchanged in 2016 compared to the year before. Across all staphylococcal species, the highest rates of resistance were seen for azithromycin (47% to 63%), oxacillin/methicillin (27% to 43%), and ciprofloxacin (25% to 30%). CoNS isolates also showed high levels of nonsusceptibility to tobramycin (20%) and trimethoprim (36%).

In 2016, besifloxacin continued to show the lowest MIC₉₀ value for both S. aureus and CoNS isolates when compared with other topical fluoroquinolones. MIC₉₀ values for besifloxacin, moxifloxacin, gatifloxacin, ciprofloxacin, and levofloxacin against S. aureus were 1, 4, 8, 64, and 32 μg/mL, respectively, and 2, 32, 32, 64, and 128 μg/mL, respectively, against CoNS isolates.

Multidrug resistance was observed in 24% of S. aureus isolates, 36% of CoNS isolates, 70% of MR-S. aureus, and 77% of MR-CoNS.

Time trends

Time trends

Because staphylococci are common causative pathogens in ophthalmic infections and considering evidence of high rates of antibiotic resistance among these organisms, a time-trend analysis was also performed to examine changes in resistance of S. aureus and CoNS isolates during the 8 years since ARMOR began. The analysis included data for 1597 S. aureus isolates and 1400 CoNS isolates submitted to ARMOR between January 2009 and October 2016.

The results showed that the overall rate of methicillin resistance among S. aureus isolates decreased significantly over time, falling from 39% in 2009 to 27% during the first 10 months of 2016. There was no decrease in the rate of methicillin resistance among CoNS isolates, and almost half of CoNS were methicillin-resistant in 2016.

Focusing on data for S. aureus isolates, the trend analysis showed significant decreases in rates of resistance from 2009 to 2016 for azithromycin (62% to 47%), ciprofloxacin (39% to 25%), and tobramycin (24% to 9%). Among CoNS isolates, resistance to ciprofloxacin also decreased significantly over the 8-year time period, from 46% in 2009 to 30% in 2016, but CoNS resistance to trimethroprim increased significantly from 26% to 37%.

Commenting on the findings of the trend analysis, Dr. Asbell told Ophthalmology Times, “Over the course of 8 years of ARMOR, staphylococci have consistently shown the highest resistance rates among the species tested.  In recent years, there has been a plateau of resistance rates, and they may be decreasing at present.  Indeed, the decreases in resistance observed among S. aureus to methicillin, macrolides, fluoroquinolones and aminoglycosides, are promising findings that may reflect improved antibiotic stewardship and an awareness of the challenges antibiotic resistance impose on patient health.” 

“However, since resistance remains high among CoNS, with no change in methicillin resistance and an increase in trimethoprim resistance, increased efforts by ophthalmologists to limit overprescribing, cycle between antibiotic classes, avoid prolonged treatment regimens, and emphasize patient dosing compliance are warranted.”

Penny Asbell, MD

E: penny.asbell@mssm.edu

ARMOR updates were presented at the 2017 meeting of the Association for Research in Vision and Ophthalmology. Dr. Asbell is a consultant to Bausch + Lomb.