title: Molecular characterization of resistance to Rifampicin in an emerging hospital-associated Methicillin-resistant Staphylococcus aureus clone ST228, Spain.
abstract: BACKGROUND
  Methicillin-resistant S. aureus (MRSA) has been endemic in Hospital Universitari de Bellvitge, Barcelona, since 1990. During the 1990-95 period the Iberian clone (ST-247; SCCmec-I) was dominant. Isolates of clonal complex 5 (ST-125; SCCmec-IV) gradually replaced the Iberian clone from 1996 to 2003. A new multiresistant MRSA phenotype showing rifampicin resistance emerged in 2004 and rapidly increased from 25% in 2004 to 45% in 2006. The aims of this study were i) the molecular characterisation of rifampicin resistant MRSA isolates, ii) the study of the rifampicin resistance expression by disk diffusion, microdilution and E-test, and iii) the analysis of the rpoB gene mutations involved in rifampicin resistance.
  
  
  RESULTS
  A sample of representative 108 rifampicin-resistant MRSA isolates belonged to a single PFGE genotype, ST-228, SCCmec type I and spa type t041. Of 108 isolates, 104 (96%) had a low-level rifampicin resistance (MICs, 2 to 4 mg/L) and 4 a high-level rifampicin resistance (MICs, 128 - > or = 256 mg/L). Disk diffusion and E-test methods failed to identify a low-level rifampicin resistance in 20 and 12 isolates, respectively. A low-level rifampicin resistance was associated with amino acid substitution 481His/Asn in the beta-subunit of RNA polymerase. Isolates with a high-level rifampicin resistance carried additional mutations in the rpoB gene.
  
  
  CONCLUSIONS
  The emergence of MRSA clone ST228-SCCmecI, related to the Southern Germany clone, involved a therapeutical challenge for treating serious MRSA infections. Decreased susceptibility to rifampicin in MRSA strains of ST228-SCCmecI was associated with one or two specific mutations in the rpoB gene. One fifth of isolates with low-level rifampicin-resistance were missed by the diffusion methods.
authors.affiliation: Microbiology Department, Hospital Universitari de Bellvitge, University of Barcelona, IDIBELL, Feixa Llarga s/n 08907 Hospitalet de Llobregat, Barcelona, Spain.
authors.forename: Virginie
authors.initials: V
authors.lastname: Mick
authors.forename: M Angeles
authors.initials: MA
authors.lastname: Domínguez
authors.forename: Fe
authors.initials: F
authors.lastname: Tubau
authors.forename: Josefina
authors.initials: J
authors.lastname: Liñares
authors.forename: Miquel
authors.initials: M
authors.lastname: Pujol
authors.forename: Rogelio
authors.initials: R
authors.lastname: Martín
mesh_terms: D000900:Anti-Bacterial Agents; D000904:Antibiotics, Antitubercular / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D015373:Bacterial Typing Techniques / Q000379:methods; D003428:Cross Infection / Q000453:epidemiology / Q000382:microbiology*; D024881:Drug Resistance, Bacterial; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D017720:Molecular Epidemiology; D009154:Mutation; D021141:Nucleic Acid Amplification Techniques / Q000379:methods; D012293:Rifampin / Q000494:pharmacology*; D013030:Spain / Q000453:epidemiology; D013203:Staphylococcal Infections / Q000453:epidemiology / Q000382:microbiology*
chemical_list: D000900:Anti-Bacterial Agents; D000904:Antibiotics, Antitubercular; D001426:Bacterial Proteins; D012293:Rifampin