A comprehensive 5-year prospective study across 72 centres in the US has shown that a significant level of antibiotic resistance exists among common ocular pathogens. The surveillance programme, entitled the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) study, is a multicenter, nationwide, prospective surveillance programme in which clinical centers across the United States were invited to submit ocular isolates of Staphylococcus aureus, coagulase-negative staphylococci (CoNS), Streptococcus pneumoniae, Haemophilus influenzae, and Pseudomonas aeruginosa. Minimum inhibitory concentrations were determined for each of the isolates by an independent central laboratory. According to the authors of the report, published in JAMA Ophthalmology, the rates of resistance among ocular pathogens does not appear to have increased over the 5-year study period however, resistance appears to be high for Staphylococcus aureus and coagulase-negative staphylococci (CoNS) strains with many isolates demonstrating multidrug resistance.
The survey organisers received a total of 3,237 ocular isolates: 1,169 S. aureus, 992 CoNS, 330 S. pneumoniae, 357 H. influenzae, and 389 P. aeruginosa) from 72 centers. Methicillin resistance (MR) was found among 493 S. aureus isolates (42.2%; 95% CI, 39.3%-45.1%) and 493 CoNS isolates (49.7%; 95% CI, 46.5%-52.9%). Resistance among S. pneumoniae isolates appeared highest for azithromycin (113 isolates [34.2%]) while resistance among P. aeruginosa and H. influenza was relatively low. As expected, staphylococcal isolates from elderly patients were more likely to be methicillin resistant. Antibiotics from 11 different classes were tested on the collected samples including, use of macrolides, aminoglycosides, lincosamides, cephalosporins, penicillins, carbapenems, dihydrofolate reductase inhibitors, polypeptides, amphenicols and andglycopeptides. Multidrug resistance to at least 3 additional antibiotic classes was found in 428 methicillin resistant S. aureus isolates (86.8%) and 381 methicillin resistant CoNS isolates (77.3%). All staphylococcal isolates were found to be susceptible to vancomycin.
Commenting on the results, the authors of the report stated that methicillin resistance continues to be high among staphylococci and that almost 50% of such resistant isolates had a high probability of also being resistant to other classes of antibiotics. In addition, the authors concluded that “the MRSA isolates were 16 to 18 times and the MRCoNS isolates were 4 to 8 times more likely to be resistant to ciprofloxacin, azithromycin, or tobramycin compared with the MS strains, and most of the MR staphylococci isolates were multidrug resistant. However, vancomycin remained active against these isolates.” Given the current diagnostic methods, which can take several days before confirmation of a particular infectious organism is possible, clinicians are recommended to consider the potential for drug resistance in respect of guiding clinical care of such infections.