Assessment of Linezolid Resistance Mechanisms Among Staphylococcus Epidermidis Causing Bacteraemia in Rome, Italy

J Antimicrob Chemother. 2010 Nov;65(11):2329-35. doi: 10.1093/jac/dkq331. Epub 2010 Sep 14.


Objectives: To characterize linezolid resistance among blood cultured Staphylococcus epidermidis from patients at the Polyclinic Agostino Gemelli (2006-08). Isolates also showed elevated MICs of macrolide, lincosamide and streptogramin (MLS) compounds, which were investigated.

Methods: Ten S. epidermidis exhibiting linezolid MICs ≥ 4 mg/L were included. Isolates were screened for cfr mutations in 23S rRNA, L3, L4 and L22, and MLS genes by PCR/sequencing. Ribosomal proteins were compared with those from a linezolid-susceptible (MIC, 1 mg/L) clinical strain and ATCC 12228. cfr location was determined by Southern blot/hybridization. The cfr strain was submitted to plasmid curing. Epidemiology was assessed by PFGE and multilocus sequence typing (MLST).

Results: S. epidermidis displayed linezolid MICs of 4 or 8 mg/L, except for strain 4303A (MIC, 64 mg/L). These organisms and a linezolid-susceptible strain exhibited L3 Leu101Val compared with ATCC 12228. Isolates also showed L3 Phe147Leu and Ala157Arg, and L4 Asn158Ser. Strain 12375A possessed L4 Lys68Arg. Isolates were wild-type for 23S rRNA and L22. cfr was plasmid located in strain 4303A and the plasmid-cured strain exhibited a linezolid MIC (4 mg/L) similar to that for cfr-negative strains (4-8 mg/L). All organisms harboured erm(A) and msr(A), while vga(A) was detected in several isolates. All isolates were clonally related and ST-23.

Conclusions: L3 Phe147Leu and/or Ala157Arg appeared responsible for the elevated linezolid MIC, since adjacent alterations have been associated with resistance. L4 Asn158Ser has been reported in a linezolid-susceptible isolate and Lys68Arg detected here did not seem to provide an additive effect. Acquisition of cfr markedly increased (8- to 16-fold) the linezolid MICs. vga(A) was associated with higher MICs of quinupristin/dalfopristin and retapamulin.

MeSH terms

  • Acetamides / pharmacology*
  • Anti-Bacterial Agents / pharmacology*
  • Bacteremia / microbiology*
  • Bacterial Proteins / genetics
  • Drug Resistance, Bacterial*
  • Hospitals
  • Humans
  • Lincosamides / pharmacology
  • Linezolid
  • Macrolides / pharmacology
  • Microbial Sensitivity Tests
  • Mutation, Missense
  • Oxazolidinones / pharmacology*
  • Point Mutation
  • RNA, Bacterial / genetics
  • RNA, Ribosomal, 23S / genetics
  • Ribosomal Proteins / genetics
  • Rome
  • Staphylococcal Infections / microbiology*
  • Staphylococcus epidermidis / drug effects*
  • Staphylococcus epidermidis / isolation & purification
  • Streptogramin B / pharmacology


  • Acetamides
  • Anti-Bacterial Agents
  • Bacterial Proteins
  • Lincosamides
  • Macrolides
  • Oxazolidinones
  • RNA, Bacterial
  • RNA, Ribosomal, 23S
  • Ribosomal Proteins
  • Streptogramin B
  • Linezolid