Staphylococcal cassette chromosome mec containing a novel mec gene complex, B4.

Objectives: To describe a new subclass of mec class B complex identified in Staphylococcus epidermidis.

Methods: Four S. epidermidis isolates obtained from bloodstream infections in patients at University Medical Center Groningen (UMCG) were analysed by phenotypic antibiotic susceptibility testing and WGS.

Results: Sequence analysis revealed a new staphylococcal cassette chromosome mec (SCCmec) structure in isolate UMCG335. In this structure, plasmid pUB110 was found to be integrated into SCCmec IVc, creating a new SCCmec subtype, IVUMCG335. SCCmec IVc and a copy of plasmid pUB110 were found in other isolates, UMCG364 and UMCG341, respectively, indicating a probability that SCCmec IVUMCG335 could have evolved at the UMCG. SCCmec of UMCG337 contained a new genetic organization of the mec complex (IS431-ΔmecR1-mecA-IS431-pUB110-IS431-ψIS1272) that we have named B4. This new subclass of mec class B complex originated by IS431-mediated inversion of the DNA segment encompassing the plasmid and most of the genes of the mec complex with the exception of IS1272. As the SCCmec organization in UMCG337 differed by the inversion of an ∼10 kb sequence compared with SCCmec IVUMCG335, we have named it SCCmec subtype IVUMCG337. Isolates UMCG335 and UMCG337 carrying SCCmec IVUMCG335 and IVUMCG337, respectively, were associated with a restriction-modification system and a CRISPR-Cas system, creating a composite island of almost 70 kb.

Conclusions: Our findings highlight the importance of IS431 in the evolution of the SCCmec region. The increasing genetic diversity identified in the SCCmec elements imposes a great challenge for SCCmec typing methods and highlights possible difficulties with the SCCmec nomenclature.