Resumo

Background Linezolid resistance has been especially mediated by mutations in the central loop of domain V of 23S rRNA among clinical staphylococcal strains since 2001, being the G2576T mutation the most prevalent. On the other hand, mutations in the ribosomal L3 protein have been associated to linezolid resistance more recently and its role in resistance development is currently under investigation. Many studies have associated L3 mutations with linezolid resistance in Staphylococcus aureus, Staphylococcus cohnii and Staphylococcus epidermidis, but not in other species of staphylococci. We report the first case of a linezolid-resistant Staphylococcus haemolyticus strain (MIC 32µg/ml) that exhibited mutations in L3 protein. Methods In May 2010, one S. haemolyticus strain exhibiting resistance to linezolid was isolated from blood culture from an inpatient in an intensive care unit of a tertiary care hospital, Brazil. The identification of the bacterial strain was performed by VITEK-2 System (bioMérieux, St. Louis, MO) and antimicrobial susceptibility testing was performed using disk diffusion and broth dilution methods according to the CLSI. Etests (AB Biodisk, Solna, Sweden) were used for oxacillin and vancomycin. The presence of the cfr gene and mutations in domain V region of 23S rRNA and rplC genes were investigated by PCR and the PCR products were sequenced and aligned with the corresponding nucleotide sequence from linezolid-susceptible S. haemolyticus strain (GenBank accession no. AP006716.1). Results The linezolid-resistant S. haemolyticus strain was multidrug-resistant, remaining susceptible to vancomycin, teicoplanin and tetracycline. The Gly139Arg and Met156Thr mutations were identified in the L3 protein of this strain, whereas the linezolid-susceptible S. haemolyticus control strain, which was recovered from a clinical sample in an inpatient from the same hospital was wild-type for L3. No mutations in 23S rRNA was identified in the linezolid-resistant S. haemolyticus strain that also did not present the cfr gene. Conclusions The identification of mutations in L3 protein without mutations in 23S rRNA in a linezolid-resistant S. haemolyticus strain strengthens the role of these sites in linezolid resistance development. Gly139Arg has already been found in linezolid-resistant S. aureus whereas Met156Thr in linezolid-resistant S. epidermidis strains, but they have not been reported in S. haemolyticus so far.