Abstract
Molecular typing is an important tool in the investigation of methicillin resistant Staphylococcus aureus (MRSA) outbreaks and in following the evolution of MRSA. The staphylococcal cassette chromosome mec (SCCmec) contains a hypervariable region with a variable number of 40 bp repeats named direct repeat units (dru). The dru region has been suggested as a supplementary typing method for MRSA and an international nomenclature exists. The purpose of this study was to investigate the diversity and variability of the dru region in a diverse collection of MRSA. We studied 302 MRSA isolates harbouring SCCmec types I to VI. The isolates represented a broad genetic background based on Staphylococcal protein A (spa) typing and multilocus sequence typing (MLST) and included 68 isolates (68 patients) from an outbreak with t024-ST8-IVa and 26 isolates from the same patient. Sequencing identified 53 dru types (dt) in 283 isolates, while eighteen isolates contained no dru repeats and one isolate resisted sequencing. The most common dru type, dt10a, was present in 53% of the sequenced isolates and was found in all SCCmec types, except type II. Seven (10%) of the 68 epidemiologically related patients had isolates with dru type variants indicating that dru typing is not useful as a first line epidemiological typing tool. However, MRSA isolates cultured from a single patient over a three year period exhibited a single dru type. The finding of dt10a in most SCCmec types suggests that dru and mecA originate from the same Staphylococcus species.
Highlights
Our understanding of methicillin resistant Staphylococcus aureus (MRSA) evolution and outbreaks has substantially increased as sequence-based typing methods have been more commonly used [1]
We studied the dru region of 302 MRSA isolates of global geographic distribution representing a broad range of genetic backgrounds and the staphylococcal cassette chromosome mec (SCCmec) types I to VI
In this study we found 53 dru types in 281 MRSA isolates suggesting a high diversity in the dru region
Summary
Our understanding of MRSA evolution and outbreaks has substantially increased as sequence-based typing methods have been more commonly used [1]. Sequencing of the Staphylococcal protein A (spa) gene is a commonly used sequence-based typing method for local MRSA outbreak investigations and has excellent interlaboratory reproducibility [2,3]. In 1991, Ryffel et al [10] described the region between IS431mec and the mecA gene in SCCmec. This region was named the hypervariable region (HVR) due to DNA length polymorphisms. Ryffel and colleagues sequenced the HVR of one MRSA strain and identified ten direct repeat units (dru) of 40 bp each. The sequencing of the dru region of 277 EMRSA-15 and EMRSA-16 led to a universal nomenclature [7,19] and since many new dru types have been reported [20,21,22]; www.dru-typing.org
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