title: Mechanisms of linezolid resistance among coagulase-negative staphylococci determined by whole-genome sequencing.
abstract: Linezolid resistance is uncommon among staphylococci, but approximately 2% of clinical isolates of coagulase-negative staphylococci (CoNS) may exhibit resistance to linezolid (MIC, ≥8 µg/ml). We performed whole-genome sequencing (WGS) to characterize the resistance mechanisms and genetic backgrounds of 28 linezolid-resistant CoNS (21 Staphylococcus epidermidis isolates and 7 Staphylococcus haemolyticus isolates) obtained from blood cultures at a large teaching health system in California between 2007 and 2012. The following well-characterized mutations associated with linezolid resistance were identified in the 23S rRNA: G2576U, G2447U, and U2504A, along with the mutation C2534U. Mutations in the L3 and L4 riboproteins, at sites previously associated with linezolid resistance, were also identified in 20 isolates. The majority of isolates harbored more than one mutation in the 23S rRNA and L3 and L4 genes. In addition, the cfr methylase gene was found in almost half (48%) of S. epidermidis isolates. cfr had been only rarely identified in staphylococci in the United States prior to this study. Isolates of the same sequence type were identified with unique mutations associated with linezolid resistance, suggesting independent acquisition of linezolid resistance in each isolate.
  
  
  OBJECTIVE
  Linezolid is one of a limited number of antimicrobials available to treat drug-resistant Gram-positive bacteria, but resistance has begun to emerge. We evaluated the genomes of 28 linezolid-resistant staphylococci isolated from patients. Multiple mutations in the rRNA and associated proteins previously associated with linezolid resistance were found in the isolates investigated, underscoring the multifocal nature of resistance to linezolid in Staphylococcus. Importantly, almost half the S. epidermidis isolates studied harbored a plasmid-borne cfr RNA methylase gene, suggesting that the incidence of cfr may be higher in the United States than previously documented. This finding has important implications for infection control practices in the United States. Further, cfr is commonly detected in bacteria isolated from livestock, where the use of phenicols, lincosamides, and pleuromutilins in veterinary medicine may provide selective pressure and lead to maintenance of this gene in animal bacteria.
authors.forename: Ryan
authors.initials: R
authors.lastname: Tewhey
authors.affiliation: Department of Laboratory Medicine, the First Affiliated Hospital of Nanjing Medical University, Nanjing, China.
authors.forename: Bing
authors.initials: B
authors.lastname: Gu
authors.affiliation: UCLA David Geffen School of Medicine, Division of Infectious Diseases, Los Angeles, California, USA.
authors.forename: Theodoros
authors.initials: T
authors.lastname: Kelesidis
authors.forename: Carmen
authors.initials: C
authors.lastname: Charlton
authors.affiliation: UCLA David Geffen School of Medicine, Department of Pathology and Laboratory Medicine, Los Angeles, California, USA.
authors.forename: April
authors.initials: A
authors.lastname: Bobenchik
authors.affiliation: UCLA David Geffen School of Medicine, Department of Pathology and Laboratory Medicine, Los Angeles, California, USA.
authors.forename: Janet
authors.initials: J
authors.lastname: Hindler
authors.affiliation: Scripps Translational Research Institute, The Scripps Research Institute, San Diego, California, USA.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Schork
authors.affiliation: UCLA David Geffen School of Medicine, Department of Pathology and Laboratory Medicine, Los Angeles, California, USA rhumphries@mednet.ucla.edu.
authors.forename: Romney M
authors.initials: RM
authors.lastname: Humphries
mesh_terms: D000081:Acetamides / Q000494:pharmacology*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001769:Blood / Q000382:microbiology; D003030:Coagulase / Q000235:genetics / Q000378:metabolism*; D024881:Drug Resistance, Bacterial; D005260:Female; D016680:Genome, Bacterial; D006801:Humans; D000069349:Linezolid; D008297:Male; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D023303:Oxazolidinones / Q000494:pharmacology*; D017422:Sequence Analysis, DNA; D013203:Staphylococcal Infections / Q000382:microbiology*; D013210:Staphylococcus / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics / Q000302:isolation & purification; D013212:Staphylococcus epidermidis / Q000187:drug effects / Q000201:enzymology / Q000235:genetics / Q000302:isolation & purification; D055815:Young Adult
chemical_list: D000081:Acetamides; D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D003030:Coagulase; D023303:Oxazolidinones; D000069349:Linezolid