Sequence based dru typing can subtype methicillin resistant Staphylococcus aureus (MRSA)

  • Mette Bartels, Department of Clinical Microbiology, Hvidovre hospital, Denmark, Denmark
  • Kit Boye, Department of Clinical Microbiology, Hvidovre hospital, Denmark, Denmark
  • Duarte Oliveira, Centro de Recursos Microbiológicos (CREM), Faculdade de Ciências e Tecnologia, Univ. Nova de Lisboa, Caparica, Portugal and Ins, Portugal
  • Henrik Westh, Department of Clinical Microbiology, Hvidovre hospital, Denmark, Denmark

    • Background
    The hypervariable region between the mecA gene and IS431 in the SCCmec cassette contains several direct repeat units (dru). We evaluated whether dru typing can be used to differentiate between MRSA isolates that are identical by other typing methods.

    Materials and methods
    dru PCR was performed on 77 International and 147 Danish MRSA isolates. Fifty-nine spa types and the SCCmec types I to VI including most subtypes and non-typeable strains were represented.
    dru PCR products were sequenced and assigned a dru sequence type (dst) based on number of repeats and SNPs.

    Results
    Nineteen isolates were dru negative. Fifty-two different dst were found in 205 isolates. Most isolates had 10 repeats (53 %) with repeat numbers ranging from 2 to14. dst10A accounted for 43 % of the isolates and was found in all SCCmec types except type II. Eleven dst (22 %) had only one nucleotide different from dst10A. Six dst (11 %) were present in two SCCmec types whereas 45 dst were found in only one SCCmec type. There was no consistency between spa type and dst.

    Conclusion
    dru-sequences are not SCCmec specific. The most common dst was found in both Danish and International MRSA strains and in all SCCmec types except type II. Interestingly, SCCmec cassettes of the same type had differences in their dru region. Altogether, these observations suggest that the dru region divergence is not linked to the SCCmec element. At the local short-term level, identical spa types could, in some cases, be discriminated by dru typing. We plan to use dru typing as a supplement to other molecular typing methods in our local epidemiology studies.