MRSA, which has been a dominant hospital-associated pathogen because of its resistance to antibiotics, is emerging in the community. Genetically, MRSA is produced when MSSA acquires a mobile genetic element staphylococcal cassette chromosome mec (SCCmec). There is considerable current interest in MRSA strains isolated from community exposure, and it has been reported that community-associated MRSA (C-MRSA) strains carry mostly type-IV SCCmec elements which are different from those carried by hospital-associated MRSA. The paper describes a novel SCCmec element (type-V SCCmec) carried by a C-MRSA strain, and anticipates a particular concern of researchers in this field by describing the characteristic features of the type-V SCCmec element in comparison with the other four types of SCCmec.

“I think the most major contribution of our research is that a new methodology for the epidemiology of MRSA has been established.”

This paper describes the fifth allotype of SCCmec (type-V SCCmec), which carried class C2 mec gene complex composed of IS431-ΔmecR1-mecA-IS431 and type-5 ccr gene complex composed of ccrC gene and surrounding orfs. In addition, the paper describes the function of the ccrC gene as a new site-specific recombinase responsible for the mobility of the type-V SCCmec element. The ccrC was proven to catalyze the precise excision as well as site- and orientation-specific integration of the type-V SCCmec element, whereas a set of ccrA and ccrB genes in other types of SCCmec elements was found to be required for those functions.

The finding of SCCmec elements leads to the notion that MRSA clones could be defined by the combination of types of SCCmec element and genotypes of MSSA strains into which the SCCmec element was integrated. SCCmec typing is now essential for the epidemiology of MRSA. By determining SCCmec types, it has been reported that MRSA clones isolated within a community were distinct from H-MRSA clones, since the former carried mostly type-IV SCCmec elements, whereas the latter carried type-I, II, and III SCCmec elements. The identification of type-V SCCmec types which does not carry any antibiotic resistance gene other than mecA in C-MRSA strains further supports the proposal that C-MRSA strains have been generated de novo from S. aureus.

I think the most important contribution of our research is that a new methodology for the epidemiology of MRSA has been established. Prior to the discovery of SCCmec elements, MRSA strains were regarded as S. aureus strains carrying mecA. However, by introducing SCCmec typing, MRSA clones could be discriminated by the other types of SCCmec elements besides genotypes. By applying this system, many studies have been reported from other countries. We have noted that well-defined C-MRSA strains carried type-IV and type-V SCCmec elements, and that large outbreaks of C-MRSA carrying type-IV SCCmec and Panton-Valentine Leukocidin genes had occurred in Uruguay.