Summary

Objectives: To test the in vitro activity of linezolid (oxazolidinone) and other antimicrobial agents against MRSA[1] isolates recovered from hospitalized patients.

Methods: We tested 150 MRSA isolates recovered from hospitalized patients. The minimal inhibitory concentration of vancomycin, teicoplanin, pristinamycin (quinupristin-dalforistin), and linezolid was determined by the Etest method. Susceptibility to other antibiotics was tested by the disk diffusion method.

Results: All isolates were sensitive to vancomycin, teicoplanin, pristinamycin, and linezolid. The MIC₉₀ was 2.0 mg/ml for vancomycin and teicoplanin (range 0.5–2.0 mg/ml and 0.125–2.0 mg/ml, respectively), and 0.5 mg/ml for pristinamycin and linezolid (range 0.125–0.75 mg/ml and 0.125–0.5 mg/m, respectively). Of the other antibiotics, fusidic acid showed the best in vitro activity, with 96.7% susceptibility, associated with trimethoprim/sulfamethoxazole (85.8%) and minocycline (84%). Penicillin was associated with the lowest susceptibility (1.3%), associated with ofloxacin (3%) and erythromycin (14%). An increase in the minimal inhibitory concentration value of vancomycin was associated with a significant decrease in resistance to TMP-SMZ[2] (P < 0.01) and an apparent increase in resistance to other antibiotics.

Conclusion: The excellent in vitro activity of linezolid and its reported in vivo effectiveness renders it an important therapeutic alternative to vancomycin in the treatment of MRSA infection.

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[1] MRSA = methicillin-resistant Staphylococcus aureus
[2] TMP-SMX = trimethoprim/sulfamethoxazole