Abstract
BackgroundAn extremely low level methicillin resistant Staphylococcus aureus (MRSA) belonging to ST45, circulates among intravenous drug users in the Zurich area. This clone can be misinterpreted as an MSSA by phenotypic oxacillin resistance tests, although it carries a staphylococcal cassette chromosome mec (SCCmec) element encoding a functional mecA gene and it produces PBP2a.ResultsThis clone carried a new 45.7-kb element, termed SCCmecN1, containing a class B mec complex (mecA-ΔmecR1::IS1272), a truncated Tn4003 harbouring the dfrA gene, and a fusB1 gene, conferring methicillin, trimethoprim and low level fusidic acid resistance, respectively. In addition to the two insertion site sequences (ISS) framing the SCCmec, a third ISS (ISS*) was identified within the element. SCCmecN1 also harboured two distinct ccrAB complexes belonging to the class 4 subtype, both of which were shown to be active and to be able to excise the SCCmecN1 or parts thereof. Slight variations in the SmaI-PFGE pattern of the clinical MRSA isolates belonging to this clone were traced back to differences in the sizes of the SCCmec J2 regions and/or to a 6.4-kb deletion extending from ISS* to the right end ISS. This latter deletion led to a variant right SCCmec-chromosomal junction site. MRSA clones carrying the shorter SCCmec with the 6.4-kb deletion were usually ciprofloxacin resistant, while strains with the complete SCCmecN1 were co-trimoxazole resistant or had no additional resistances. This suggested that the genetic backbone of the host S. aureus, although identical by PFGE pattern, had at some stage diverged with one branch acquiring a sulfonomide resistance mutation and the other ciprofloxacin resistance.ConclusionThis description of the structure and variations of SCCmecN1 will allow for quicker and easier molecular detection of this clone and monitoring of its spread.
Highlights
An extremely low level methicillin resistant Staphylococcus aureus (MRSA) belonging to ST45, circulates among intravenous drug users in the Zurich area
This is larger than the community-associated staphylococcal cassette chromosome mec (SCCmec) type IV (21–25-kb), type V (27.6-kb) and type VI elements, falling within the range of the classical hospital associated SCCmec types I-III which range in size from 34–67 kb [23]
Mec complex typing SCCmec typing [20] results suggested that the mec complex did not contain mecI and PCR using primers spanning IS1272 and ΔmecR1 and sequencing over the gene junction confirmed the presence of a class B mec complex
Summary
An extremely low level methicillin resistant Staphylococcus aureus (MRSA) belonging to ST45, circulates among intravenous drug users in the Zurich area. The transmission of MRSA clones through both communityand healthcare-associated routes is responsible for the high incidence of soft tissue infections and increases in severe infections such as endocarditis and bacteremia in IDUs [6,7,8] Such a clonal dispersal led to MRSA becoming endemic in the Zurich IDU population, where in 2003 24% of all MRSA isolates collected at the University hospital of Zurich belonged to a single, so called "drug clone" [9]. Difficulties arise from strains expressing lowlevel but heterogenous resistance, that upon exposure to β-lactams segregate highly resistant subpopulations resulting in therapy failure [11] Misdiagnosis of such strains with very low, phenotypically susceptible, minimum inhibitory concentrations (MICs) is a major problem necessitating the use of molecular detection methods
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