Multidrug Resistance Common Among Eye Pathogens, Study Finds

Nicola M. Parry, DVM

October 27, 2015

Antibiotic resistance continues to be high among ocular pathogens. Nearly half of staphylococcal isolates are resistant to methicillin, and most of those isolates are also resistant to multiple other commonly used ophthalmic antibiotics, according to data from the prospective surveillance Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) study.

Penny A. Asbell, MD, MBA, a professor of ophthalmology at the Icahn School of Medicine at Mount Sinai School, New York City, and colleagues published the results of their study online October 22 in JAMA Ophthalmology.

"ARMOR is designed to provide healthcare professionals with information about evolving trends in antimicrobial susceptibility patterns to help guide treatment decisions," Dr Asbell told Medscape Medical News. "Knowing that resistance among staphylococci can be associated with multidrug resistance is important information for all clinicians and should help in selecting initial antibiotic treatment, before culture results, if [they are] done, are available."

Although trends in antibiotic resistance among ocular bacterial pathogens have been reported from single-center studies, data from larger studies are rare. Dr Asbell and colleagues therefore initiated the multicenter, nationwide, prospective ARMOR study in 2009. "The only surveillance study on ocular infections is the ARMOR study, and this specifically looks for trends in antibiotic resistance among common ocular pathogens and common topical antibiotics used to treat eye infections," Dr Asbell said.

The survey included organisms most frequently implicated in bacterial eye infections, including Staphylococcus aureus, Streptococcus pneumoniae, coagulase-negative staphylococci (CoNS), Pseudomonas aeruginosa, and Haemophilus influenzae.

From 2009 to 2013, the researchers analyzed 3237 bacterial isolates (1169 S aureus, 992 CoNS, 330 S pneumoniae, 357 H influenzae, and 389 P aeruginosa) from 72 participating centers across 36 states. Antibiotics from different classes, including fluoroquinolones, aminoglycosides, macrolides, cephalosporins, penicillins, dihydrofolate reductase inhibitors, amphenicols, and glycopeptides, were tested against the bacterial isolates at a central laboratory.

Methicillin resistance was found among 493 S aureus isolates (resistance rate [RR], 42.2%; 95% confidence interval [CI], 39.3% - 45.1%) and 493 CoNS isolates (RR, 49.7%; 95% CI, 46.5% - 52.9%). Methicillin-resistant (MR) isolates were also more likely to be resistant to other ophthalmic antibiotics, including fluoroquinolones, aminoglycosides, or macrolides (P < .001); multidrug resistance to at least three additional antibiotic classes occurred in 428 MR S aureus isolates (86.8%) and 381 MRCoNS isolates (77.3%).

Staphylococcal and CoNS isolates obtained from elderly patients, and staphylococcal isolates obtained from patients from the southern United States, were also more likely to be MR (P < .001 for all). Among S pneumoniae isolates, resistance was highest for azithromycin (113 isolates [34.2%]).

However, all staphylococcal isolates were susceptible to vancomycin, and only a small proportion were resistant to trimethoprim (46 [4.7%] of 969 isolates) and chloramphenicol (4 [0.4%] of 969 isolates). Most of the 330 S pneumoniae isolates were susceptible to antibiotics tested, including moxifloxacin (329 [99.7%] of 330 isolates) and chloramphenicol (321 [97.3%] of 330 isolates). All CoNS isolates were also susceptible to vancomycin. Among P aeruginosa isolates, resistance rates to the antibiotics tested were low, with good susceptibility to fluoroquinolones such as ciprofloxacin (359 [92.3%] of 389 isolates) and tobramycin (377 [96.9%] of 389 isolates). All except two H influenzae isolates were susceptible to all antibiotics tested.

During the 5-year study period, methicillin resistance among staphylococci did not increase (P ≤ .22), and resistance to ciprofloxacin among CoNS and MRCoNS isolates, and to tobramycin among CoNS isolates, slightly decreased (P ≤ .03).

"While our finding that there was no increase in methicillin resistance observed among staphylococci isolates over the 5 years was surprising, the data demonstrate that methicillin resistance remained prevalent, with many strains demonstrating multidrug resistance," Dr Asbell told Medscape Medical News.

"The ARMOR study is helpful in identifying risk groups and gives doctors a better idea of how to start treatment in patients at risk for resistant infections. With that said, clinicians must consider each case individually," she noted. "The effect of treatment often depends upon a combination of factors, including the pathogen, the choice of antibiotic, and importantly, the patient."

According to Dr Asbell, "understanding best practices for antibiotic use should help maximize the effectiveness of antibiotic use and decrease the risk of contributing to the growing problem of resistance to antibiotics." The ARMOR study "is now in its seventh consecutive year, and plans are underway to continue surveillance research. Continued vigilance is warranted to monitor long-term patterns of drug resistance among bacterial pathogens that are prevalent in ocular infections," she concluded.

This study was supported by Bausch & Lomb. Dr Asbell is an advisory board member for Valeant/Bausch & Lomb. The other authors have disclosed no relevant financial relationships.

JAMA Ophthalmol. Published online, October 22, 2015. Full text


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