The staphylococcal cassette chromosome (SCC) is a gene cassette widely disseminated in staphylococci. SCCmec is specified by the carriage of methicillin resistant- determinant (mecA). MSSA changes to MRSA upon the acquisition of SCCmec. When the SCCmec excises from the MRSA chromosome, MRSA changes to MSSA.
Structural comparison of SCCmec elements.
SCCmec elements commonly share the following characteristics.
(1) They carry a mec gene complex (mec).
(2) They carry a ccr gene complex (ccr).
(3) They have characteristic directly-repeated nucleotide sequences and inverted-complementary sequences at both ends.
(4) They integrate into ISS (integration site sequence for SCC), which is located at 3’ end of orfX.
In the structure of SCCmec elements, the regions other than ccr gene complex (ccr) and mec gene complex (mec) are designated junkyard (J) regions. The junkyard regions comprise three parts: J1 (the region between ccr and the chromosomal region flanking SCCmec), J2 (the region between mec and ccr), and J3 (the region between orfX and mec).
Diversities in the structure of SCCmec elements.
Despite the similarities described above, the structure of SCCmec elements are rather divergent. Allotypic differences have been identified in both ccr and mec. Four classes of mec and five types of ccr have been reported. Furthermore, the numerous differences in J regions have been identified. The SCCmec elements have been classified based on the differences in mec, ccr, and for subtyping, also the J regions.
(1) SCCmec type: defined by the combination of mec and ccr.
|SCCmec type||combination of ccr and mec|
|Type I||type-1 ccr carrying ccrA1 and B1 and class B mec (1B)|
|Type II||type-2 ccr carrying ccrA2 and B2 and class A mec (2A)|
|Type III||type-3 ccr carrying ccrA3 and B3 and class A mec (3A)|
|Type IV||type-2 ccr carrying ccrA2 and B2 and class B mec (2B)|
|Type V||type-5 ccr carrying ccrC and class C mec (5C)|
|Type VI||type-4 ccr carrying ccrA4 and B4 and class B mec (4B)|
(2) Subtypes : defined by the structural differences in J1, -2 and -3 regions
The J regions are not always specific to each SCCmec type, but certain J regions are commonly shared among certain types of SCCmec elements. Of the three regions, we regard J1 as the most fundamental, because we presume that it reflects the original form of SCC into which a mec gene complex have been integrated. Moreover, several different J1 regions have been identified in type-II and type-IV SCCmec elements. The presence or absence of integrated plasmids encoding drug resistance genes in the J3 regions of SCCmec elements can also be used as markers to classify SCCmec elements further in epidemiological studies.
Illustrations of SCCmec elements that link to data bases.
Please click SCCmec Type to see detailed structure of the element or to see subtypes of each SCCmec type.
On the description of the SCCmec types.
We proposed a new nomenclatural system for SCCmec elements that has three features. The first feature is a description of SCCmec type. A SCCmec element can be simply described by ccr type (indicated by a number) and mec class (indicated by an uppercase letter). Alternatively, Roman numeral designations can be used instead of combinations of ccr and mec（Chongtrakool et al. 2006）. We retained Roman numeral to describe SCCmec type as feedback from a large number of researchers who told us that they were convenient and well established. We have developed a new nomenclature for SCCmec types, which is an attempt to resolve confusion in the field and to facilitate the identification of novel SCCmec elements. In 2001, Oliveira et al. reported the a pediatric strain, HDE288, carried novel SCCmec element of which ccr genes were not identical to three previously reported ccr genes, and designated the new SCCmec element as type-IV SCCmec（Oliveira et al, 2001）. Independently, Ma et al. have found SCCmec elements carrying class B mec and type-2 ccr in C-MRSA strains, which were isolated at the University of Chicago Children’s Hospital, and designated these type-IV SCCmec (Ma et al., 2002). Since MRSA strains carrying the latter elements are found mostly in C-MRSA strains isolated in the USA or Europe, many researchers have regarded elements carrying type-2 ccr and class B mec as type-IV SCCmec. If the proposed nomenclature were adopted, the type-IV SCCmec element found in HD288 would be called type-4B, signifying that the element carried type-4 ccr (fourthly identified ccr) and class B mec. As an alternative to Roman numerals, the element carried by HDE288 would be the prototypic type IV, since it was reported in 2001. However, at present, type-IV SCCmec is regarded as an element that carries class B mec and type-2 ccr, and type-V SCCmec as an element carrying class C mec and type-5 ccr carrying ccrC. Therefore, it is agreed that the SCCmec element that is carried by HDE288 should be regarded as type VI SCCmec, since it carries a sixth ccrand mec combination, consisting of type-4 ccr and class B mec. The second feature of our nomenclatural system is a description of J regions, which could be indicated by following the SCCmectype designation with numbers representing differences in the J1, J2, and J3 regions, each separated by a period. The subtypes of type-II and type-IV SCCmec elements would be described in numerals in the order that they are found, as follows (previous names are indicated in parentheses): type II.1 (type IIa), type II.2 (type IIb), type IV.1 (type IVa ), type IV.2 (type IVb), type IV.3 (type IVc), and type IV.4 (type IVd). The different J regions were distinguished by numbering them. The third feature is the numbering of SCCmec types and their subtypes: the number should be given in chronological order of them being discovered.
The description of J regions has been modified from our original proposal in line with the suggestions from Dr. Kunyan Zhang (University of Calgary, Canada).