Resistance to linezolid is mediated by the cfr gene in the first report of an outbreak of linezolid-resistant Staphylococcus aureus.
BACKGROUND
From April through June 2008, we identified 12 patients in the intensive care unit and 3 patients on other wards infected with methicillin-resistant Staphylococcus aureus that was also resistant to linezolid. We investigated the mechanism of resistance--point mutations in domain V of 23S ribosomal RNA (rRNA) or presence of the cfr gene--involved in the outbreak.


METHODS
Strains for the study were obtained in the intensive care unit and other wards. Minimal inhibitory concentrations were determined using automated methods, the E-test, or dilution in Mueller-Hinton agar in accordance with Clinical and Laboratory Standards Institute guidelines. Strains were genotyped using pulsed-field gel electrophoresis and were sequenced to determine the presence of point mutations in 23S rRNA. The presence of the cfr gene was determined by specific polymerase chain reaction.


RESULTS
The minimal inhibitory concentrations of linezolid ranged from 16 mg/L to 32 mg/L, and all the strains were susceptible to tigecycline, vancomycin, and daptomycin. Typing of strains sequentially isolated by pulsed-field gel electrophoresis showed that each patient carried only 1 clonal type of linezolid-resistant, methicillin-resistant S. aureus as detected by sequential isolations. The presence of the cfr gene was confirmed in all the isolates. Furthermore, sequencing of domain V of 23S rRNA showed that the most common mechanism of linezolid resistance reported to date, mutation G2576T, was not detected in any of the strains analyzed.


CONCLUSIONS
We report the presence of the cfr gene underlying the resistance mechanism involved in a clinical outbreak of linezolid-resistant S. aureus.
Clinical Microbiology Service, Hospital Clínico San Carlos, Madrid, Spain. gmorales.hcsc@salud.madrid.org
Gracia
G
Morales
Juan J
JJ
Picazo
Elvira
E
Baos
Francisco J
FJ
Candel
Ana
A
Arribi
Beatriz
B
Peláez
Raquel
R
Andrade
María-Angeles
MA
de la Torre
José
J
Fereres
Miguel
M
Sánchez-García
D000081:Acetamides / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics*; D015373:Bacterial Typing Techniques; D016172:DNA Fingerprinting; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D004275:DNA, Ribosomal / Q000737:chemistry / Q000235:genetics; D004196:Disease Outbreaks; D024881:Drug Resistance, Bacterial; D016521:Electrophoresis, Gel, Pulsed-Field; D005838:Genotype; D006801:Humans; D000069349:Linezolid; D055624:Methicillin-Resistant Staphylococcus aureus / Q000145:classification / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D023303:Oxazolidinones / Q000494:pharmacology*; D016133:Polymerase Chain Reaction; D012338:RNA, Ribosomal, 23S / Q000235:genetics; D017422:Sequence Analysis, DNA; D013203:Staphylococcal Infections / Q000453:epidemiology / Q000382:microbiology*
D000081:Acetamides; D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; C539943:CFR protein, Staphylococcus aureus; D004269:DNA, Bacterial; D004275:DNA, Ribosomal; D023303:Oxazolidinones; D012338:RNA, Ribosomal, 23S; D000069349:Linezolid