Novel staphylococcal cassette chromosome mec (SCCmec) type XIV (5A) and a truncated SCCmec element in SCC composite islands carrying speG in ST5 MRSA in Japan

Background: Staphylococcal cassette chromosome mec (SCCmec) elements are highly diverse and have been classified into 13 types. The arginine catabolic mobile element (ACME) is an SCC-Like element harbouring an arginine deiminase pathway gene cluster (ACME-arc). ACME type I (ACME I), additionally including a spermidine/ spermine-N-1-acetyltransferase gene (speG), is considered to have contributed to the rapid spread of the most successful MRSA clone, USA300. Objectives: To characterize the SCC composite islands (SCC-CIs) in ST5 MRSA positive for both ACME-arc and speG. Methods: Three ST5 MRSA strains (SC640, SC792 and SC955) collected in Hokkaido, Japan were subjected to WGS and the SCC-CIs were determined. Results: The SCC-CIs consisted of four (SC640 and SC792) or three (SC955) SCC/SCC-Like elements and commonly harboured both an ACME type II' and an SCC encoding speG. These SCC-CIs appear to mimic ACME I in USA300, in that they are equipped with ACME-arc and speG. The SCC-CIs of SC640 and SC792 contained novel SCCmec/ SCCmec-Like elements at the 3' end, whereas SC955 contained SCCmec type V. The SCCmec of SC792 carried mec complex A and ccrC1, which was determined to be novel and designated as SCCmec type XIV (5A). SC640 harboured an SCCmec-Like element derived from SCCmec type XIV. It Lacked most of the downstream region of the mec complex, including the Left chromosomal attachment site (SCCmec XIV Akdp/DR-L), and Lost its capability for chromosomal excision, suggesting that the mecA gene is immobilized on the chromosome. Conclusions: These findings provide evidence for increasing complexity of SCC-CIs.