This study aimed at determining the mechanisms of linezolid resistance and the molecular characteristics of clinical Staphylococcus aureus (n = 2) and coagulase-negative staphylococci (n = 15) isolates obtained from four Spanish hospitals. The detection of linezolid resistance mechanisms (mutations and acquisition of resistance genes) was performed by PCR/sequencing. The antimicrobial resistance and virulence profile was determined, and the isolates were typed by different molecular techniques. Moreover, the genetic environment of the cfr gene was determined by whole-genome sequencing. The cfr gene was detected in one methicillin-resistant S. aureus (MRSA) that also displayed the amino acid change Val118Ala in the ribosomal protein L4. The second S. aureus isolate was methicillin susceptible and showed different alterations in the ribosomal protein L4. All remaining linezolid-resistant Staphylococcus epidermidis (n = 14) and Staphylococcus hominis isolates (n = 1) showed the mutation G2576T (n = 14) or C2534T (n = 1) in the 23S rRNA. Moreover, different amino acid changes were detected in the ribosomal proteins L3 and L4 in S. epidermidis isolates. All S. epidermidis isolates belonged to the multilocus sequence type ST2. Linezolid-resistant staphylococci (LRS) showed a multiresistance phenotype, including methicillin resistance that was detected in all isolates but one, and was mediated by the mecA gene. The cfr gene in the MRSA isolate was located together with the fexA gene on a conjugative 38,864 bp plasmid. Linezolid- and methicillin-resistant S. epidermidis ST2 showing mutations in the 23S rRNA and in the ribosomal proteins L3 and L4 are spread among Spanish hospitals, whereas LRS carrying acquired linezolid resistance genes are rarely detected.
Keywords: S. aureus; S. epidermidis; ST2; cfr; linezolid.