MRSA detection: comparison of two molecular methods (BD GeneOhm® PCR assay and Easy-Plex) with two selective MRSA agars (MRSASelect ® and Oxoid MRSA) for Nasal swabs

  • Sebastiaan van Hal, Department of Microbiology and infectious diseases, St.Vincent's Hospital, Sydney, Australia, Australia
  • Miss Zoe Jennings, Department of Microbiology and infectious diseases, St.Vincent's Hospital, Sydney, Australia, Australia
  • Dr Damien Stark, Department of Microbiology and infectious diseases, St.Vincent's Hospital, Sydney, Australia, Australia
  • A/Prof Debbie Marriott, Department of Microbiology and infectious diseases, St.Vincent's Hospital, Sydney, Australia, Australia
  • A/Prof Jock Harkness, Department of Microbiology and infectious diseases, St.Vincent's Hospital, Sydney, Australia, Australia

    • Screening of patients is an important component for methicillin-resistant Staphylococcus aureus (MRSA) containment. The aim of this study was to determine the utility of molecular methods compared to selective agars for MRSA detection
    Consecutive high risk patients (n=200) were screened for nasal MRSA colonization using chromogenic selective agars (MRSA ID® and CMRSA) and the BD GeneOhm MRSA (previously known as IDI-MRSA) and the multiplex tandem real time Easy-Plex PCR assays. A gold standard was defined as ≥ 2 methods positive with discordant results resolved by alternative methods. All isolates were typed using pulse field gel electrophoresis and coagulase gene RFLP and compared to fully typed MRSA clones (multi-locus sequence typing).
    Ninety true positive MRSA isolates were identified representing a colonization rate of 45% (90/200). The sensitivities of the molecular methods were similar with BD GeneOhm MRSA (86%) and Easy-Plex (84%). The chromogenic agars, MRSA ID® and CMRSA showed sensitivities of 62% and 84% respectively. Sensitivity for all methods was influenced by the MRSA antibiograms with better detection of non-multi-resistant MRSA. Similarly, MRSA clonal type influenced the detection rate. Aus EMRSA-2 (corrseponding to ST239-MRSA-III) and the Queensland clone (corresponding to ST93-MRSA-IV) were the poorest detected clones; 73% and 67% of the time respectively. The specificity was >98% in all methods except for the Easy-Plex assay (90%). Sensitivity of selective agars increased with 48 hours of incubation with a corresponding decrease in specificity.
    In conclusion, molecular detection methods for MRSA remain sensitive and rapid but are associated with greater expense.