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C. N. Ta, D. F. Sahm, Ocular TRUST; Antimicrobial Susceptibility Patterns in MRSA vs MSSA Eye Infection: Results from Ocular TRUST. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2682.
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Broad-spectrum antibiotics such as the newer fluoroquinolones (FQs) levofloxacin, gatifloxacin, and moxifloxacin are valued for their activity against a wide range of Gram-positive and -negative pathogens, including Staphylococcus aureus (SA). However, methicillin-resistant (MRSA) strains are often simultaneously resistant to multiple antibiotics, including FQs. Longitudinal surveillance to detect changes in resistance can facilitate therapy selection. We report antimicrobial susceptibilities in ocular MRSA and MSSA (methicillin-susceptible) isolates from the ongoing, nationwide Ocular TRUST surveillance study.
SA isolates from eye infections were prospectively collected between October 2005 and June 2006 from 35 geographically distributed U.S. community and eye hospitals. All isolates were tested by broth microdilution according to CLSI methodology at a centralized independent laboratory. Microtiter panels included common ophthalmic antibiotics. MICs were interpreted as susceptible, intermediate, or resistant according to 2006 CLSI criteria.
197 SA isolates were evaluated for methicillin susceptibility. Of 164 MSSA strains, 81.1% were susceptible to levofloxacin, gatifloxacin, and moxifloxacin. However, only 15.2% of 33 MRSA isolates were susceptible to these three FQs. Ciprofloxacin susceptibilities were similar: 79.9% of MSSA vs.15.2% of MRSA. Highest susceptibility was seen with trimethoprim (93.9% of MRSA). MRSA isolates were 100% resistant to both penicillin and polymyxin B. Rank order of MRSA susceptibility to all antimicrobials tested: trimethoprim > tobramycin > levofloxacin = gatifloxacin = moxifloxacin = ciprofloxacin > penicillin = polymyxin B.
The newer FQs (levofloxacin, gatifloxacin, moxifloxacin) have equivalent activity against MSSA (81.1%) and MRSA (15.2%) ocular isolates. Surveillance confirms that MRSA is a multidrug-resistant pathogen in ocular infections; ≥64% of MRSA isolates were resistant to all antimicrobials tested except trimethoprim. When selecting therapy in MRSA infection, the potential for antibiotic failure must be considered.
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