title: Assessment of linezolid resistance mechanisms among Staphylococcus epidermidis causing bacteraemia in Rome, Italy.
abstract: OBJECTIVE
  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.
authors.affiliation: JMI Laboratories, North Liberty, IA 52317, USA. rodrigo-mendes@jmilabs.com
authors.forename: Rodrigo E
authors.initials: RE
authors.lastname: Mendes
authors.forename: Lalitagauri M
authors.initials: LM
authors.lastname: Deshpande
authors.forename: David J
authors.initials: DJ
authors.lastname: Farrell
authors.forename: Teresa
authors.initials: T
authors.lastname: Spanu
authors.forename: Giovanni
authors.initials: G
authors.lastname: Fadda
authors.forename: Ronald N
authors.initials: RN
authors.lastname: Jones
mesh_terms: D000081:Acetamides / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016470:Bacteremia / Q000382:microbiology*; D001426:Bacterial Proteins / Q000235:genetics; D024881:Drug Resistance, Bacterial; D006761:Hospitals; D006801:Humans; D055231:Lincosamides / Q000494:pharmacology; D000069349:Linezolid; D018942:Macrolides / Q000494:pharmacology; D008826:Microbial Sensitivity Tests; D020125:Mutation, Missense; D023303:Oxazolidinones / Q000494:pharmacology*; D017354:Point Mutation; D012329:RNA, Bacterial / Q000235:genetics; D012338:RNA, Ribosomal, 23S / Q000235:genetics; D012269:Ribosomal Proteins / Q000235:genetics; D012384:Rome; D013203:Staphylococcal Infections / Q000382:microbiology*; D013212:Staphylococcus epidermidis / Q000187:drug effects* / Q000302:isolation & purification; D025381:Streptogramin B / Q000494:pharmacology
chemical_list: D000081:Acetamides; D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D055231:Lincosamides; D018942:Macrolides; D023303:Oxazolidinones; D012329:RNA, Bacterial; D012338:RNA, Ribosomal, 23S; D012269:Ribosomal Proteins; D025381:Streptogramin B; D000069349:Linezolid