Linezolid was the first member of the family of oxazolidinones to be introduced in clinical practice. This antibiotic has proven to be very effective against most of the Gram-positive organisms, including vancomycin-resistant or vancomycin-intermediate methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus sp.1 Development of resistance was described only a few years after its introduction in clinical practice.2, 3 Development of resistance in these reports was due to G2576T mutation in the 23S rRNA gene, and it appeared to be an uncommon finding.2, 3, 4 However, the first outbreak of a linezolid-resistant MRSA due to a different resistance mechanism has recently been described: the acquisition of cfr gene, which also confers chloramphenicol, 16C macrolides, streptogramin A and clindamycin resistance.5 Previously the cfr gene was described in other staphylococcal species of animal origin.6 Moreover, other cases of cfr-mediated linezolid resistance in S. aureus and S. epidermidis strains isolated from human infections have also been published.4, 7 Here we report a case of acquisition of linezolid resistance from MRSA that was related to the development of chloramphenicol/clindamycin resistance with the parallel isolation of one linezolid-resistant S. epidermidis strain.

The patient was a 73-year-old woman who underwent an aortic valve replacement. Three weeks after surgery, the patient experienced fever, inflammatory symptoms in the surgical wound, thoracic instability, and cardiac and renal failure. The patient was then diagnosed with acute mediastinitis, and a linezolid-susceptible MRSA strain was isolated from two blood cultures (day 1). The patient was then treated with linezolid (600 mg b.i.d. intravenous), and day 18, medication was changed to oral therapy with clindamycin (300 mg b.i.d.) and linezolid (600 mg b.i.d.). Two weeks (day 32) after this modification, the disease persisted, and new cultures were performed (one from a wound exudate and four blood cultures). Linezolid-resistant MRSA strains were isolated from a wound exudate and from two blood cultures samples, and the linezolid-resistant strain of S. epidermidis was isolated from one blood culture mixed with one linezolid-resistant MRSA and in two additional blood cultures in pure culture.

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