title: Evaluation of commercial chromogenic media for the detection of meticillin-resistant Staphylococcus aureus.
abstract: BACKGROUND
  Selective chromogenic media allowing one-step meticillin-resistant Staphylococcus aureus (MRSA) isolation and identification are widely used. However, the changing epidemiology of MRSA means that the suitability of these chromogenic media requires investigation.
  
  
  OBJECTIVE
  To evaluate the following chromogenic media - Colorex MRSA, MRSA Select II, ChromID MRSA, and MRSA Brilliance 2 - for the detection of divergent strain types.
  
  
  METHODS
  We used a diverse collection of S. aureus, including strains harbouring the mecC gene, strains expressing varying levels of meticillin resistance, and isolates recovered from patient samples.
  
  
  RESULTS
  MRSA Select II, Colorex MRSA, and ChromID each grew at a density of 1.5 × 10(1)cfu/mL for each SCCmec type investigated. Brilliance 2 demonstrated growth at 1.5 × 10(1)cfu/mL for mecC MRSA but at a higher density (1.5 × 10(4)cfu/mL) for the three mecA MRSA strains. All four media demonstrated excellent sensitivity for MRSA detection (≥99%), but reduced levels of specificity (85-73%) when challenged with a range of meticillin-susceptible S. aureus (MSSA) isolates. High levels of false positives (∼50%) were also obtained with all chromogenic media when tested with mec-negative borderline oxacillin-resistant S. aureus (BORSA) isolates.
  
  
  CONCLUSIONS
  Although false positives may be obtained with some strains of MSSA and BORSA, the high sensitivity of these media and their ability to recover almost all MRSA tested (including oxacillin-susceptible and mecC-positive strains) confirm the value of chromogenic agar in MRSA detection.
authors.affiliation: National MRSA Reference Laboratory, St. James's Hospital, Dublin, Ireland; Microbiology Research Unit, Dublin Dental University Hospital, University of Dublin, Trinity College Dublin, Ireland. Electronic address: gbrennan@stjames.ie.
authors.forename: G I
authors.initials: GI
authors.lastname: Brennan
authors.affiliation: School of Biological Sciences, Dublin Institute of Technology, Dublin, Ireland.
authors.forename: C
authors.initials: C
authors.lastname: Herra
authors.affiliation: Microbiology Research Unit, Dublin Dental University Hospital, University of Dublin, Trinity College Dublin, Ireland.
authors.forename: D C
authors.initials: DC
authors.lastname: Coleman
authors.affiliation: National MRSA Reference Laboratory, St. James's Hospital, Dublin, Ireland; Department of Clinical Microbiology, School of Medicine, University of Dublin, Trinity College, St. James's Hospital, Dublin, Ireland.
authors.forename: B
authors.initials: B
authors.lastname: O'Connell
authors.affiliation: National MRSA Reference Laboratory, St. James's Hospital, Dublin, Ireland; Microbiology Research Unit, Dublin Dental University Hospital, University of Dublin, Trinity College Dublin, Ireland.
authors.forename: A C
authors.initials: AC
authors.lastname: Shore
mesh_terms: D001431:Bacteriological Techniques / Q000379:methods*; D002863:Chromogenic Compounds / Q000378:metabolism*; D003470:Culture Media / Q000737:chemistry*; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000302:isolation & purification* / Q000378:metabolism*; D012680:Sensitivity and Specificity
chemical_list: D002863:Chromogenic Compounds; D003470:Culture Media
keywords: Borderline oxacillin-resistant S. aureus chromogenic agar; Meticillin-resistant Staphylococcus aureus; mecC