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CHICAGO -- Antimicrobial resistance profiles in ocular isolates over the past decade show continued high antimicrobial resistance among staphylococci and increased resistance among Streptococcus pneumonia to azithromycin.

Trends in resistance profiles of ocular staphylococcal and pneumococcal bacteria were presented by from the Icahn School of Medicine at Mount Sinai, New York City, at the 2016 meeting.

She used two nationwide surveillance studies performed roughly a decade apart -- (2006-2008) and (2009-2015) -- to examine antibiotic resistance profiles and resistance trends over time.

"TRUST and ARMOR together give us a big picture of what's going on in ocular isolates in the United States, and methicillin resistance is definitely there," said Asbell.

ARMOR is the only ongoing surveillance study specific to pathogenic ocular bacteria, concentrating on gram-positive organisms, which constitute 60% or more of bacterial infections encountered in ophthalmology, she said. Among the isolates in the ARMOR survey are isolates of S. pneumonia, Staphylococcus aureus, and coagulase-negative staphylococci.

"We combined results from TRUST and ARMOR to look at what's happening with regard to resistance rates over the past decade," Asbell told MedPage Today. "The thing that made us think of surveillance studies is that earlier data from 2000 to 2005, when you look at ocular S. aureus, over time you see a growing level of methicillin resistance, and we didn't know where it was going." Over that time, ocular MRSA jumped from about 30% to more than 40%, and the MRSA ocular isolates were multidrug-resistant, defined as in vitro resistance to at least three antibiotic agents, including all fluoroquinolones tested.

For both studies, minimum inhibitory concentrations (MICs) for the antibiotic classes were determined by broth microdilution according to Clinical and Laboratory Standards Institute guidelines.

In total, more than 5,000 isolates were analyzed: 1,229 from Ocular TRUST and 4,226 from ARMOR. "When you look at the resistance patterns, what you see is some degree of resistance to almost all of the topical agents we use in ophthalmology," said Asbell. "It varies from product to product but it exists."

Among the findings:

• Of the 1,991 S. aureus isolates analyzed, 38% were methicillin-resistant.
• Of the 1,456 isolates of coagulase-negative staphylococci, 50% were methicillin-resistant.
• Of the 789 S. pneumonia isolates, 34% were resistant to azithromycin.

"The best of the drugs that we currently use for S. aureus might be trimethoprim followed by tobramycin," she said. Fewer than 10% of S. aureus and methicillin-resistant S. aureus (MRSA) were resistant to trimethoprim. About 20% and 40%, respectively, were resistant to tobramycin.

Among coagulase-negative staphylococci, resistance to all antibiotics was again observed, with tobramycin performing best, with fewer than 20% of coagulase-negative staphylococci and fewer than 30% of methicillin-resistant coagulase-negative staphylococci isolates showing resistance to tobramycin.

Resistance to each of the antibiotics over the 10-year period was relatively stable, with a few exceptions. A small but significant decrease was found in S. aureus resistance (P<0.05) against oxacillin, ciprofloxacin, and tobramycin, and in coagulase-negative staphylococci, resistance (P<0.05) against oxacillin and ciprofloxacin.

For both S. aureus and coagulase-negative staphylococci, in-vitro MIC90 values were nearly as low with besifloxacin as with vancomycin, the gold standard, indicating high potency. "MIC90 values for ciprofloxacin and tobramycin were significantly higher than the other fluoroquinolones, and certainly higher than vancomycin," she said. "When we're picking an antibiotic, we want potency and penetration, especially if the infection is deeper into the eye tissue."

Over the past 2 years, there seemed to be a slight decrease in MRSA and resistant coagulase-negative staph, "maybe suggesting that the long-term is we're going to see a decline, but you won't know unless you do studies," Asbell said. Resistance among S. pneumonia to azithromycin, however, doubled over the 10-year period, from 22% to 44% (P<0.05).

"Surveillance is a good way of getting a big picture and does help a clinician to pick an initial treatment," she said. "If they're going to do a culture, it may change depending on culture results, but it gives them a big picture."

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