SCC''mec'', or staphylococcal cassette chromosome ''mec'', is a

mobile genetic element Mobile genetic elements (MGEs), sometimes called selfish genetic elements, are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...

of ''

Staphylococcus ''Staphylococcus'', from Ancient Greek σταφυλή (''staphulḗ''), meaning "bunch of grapes", and (''kókkos''), meaning "kernel" or " Kermes", is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillale ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

l species. This genetic sequence includes the ''mecA'' gene coding for resistance to the

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...

methicillin Methicillin ( USAN), also known as meticillin ( INN), is a narrow-spectrum β-lactam antibiotic of the penicillin class. Methicillin was discovered in 1960. Medical uses Compared to other penicillins that face antimicrobial resistance ...

and is the only known way for ''Staphylococcus'' strains to spread the gene in the wild by

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...

. SCC''mec'' is a 21 to 60 kb long genetic element that confers broad-spectrum β-lactam resistance to MRSA. Moreover, additional genetic elements like Tn''554'', pT181, and pUB110 can be found in SCC''mec,'' which have the capability to render resistance to various non-β-lactam drugs.

Classification

Not all SCC''mec'' elements are identical (in fact, SCC elements without the ''mecA'' gene do exist.) As of December 2021, ''SCCmec'' elements have been classified into fourteen types (I through XIV). One region is the ''mec'' complex including the ''mecA'' gene. The other is the ''ccr'' gene complex including genes coding for recombinases. The ''mec'' complex is divided further into five types (I through V) based on the arrangement of regulatory genetic features such as ''mecR1'', an inducer. The ''mec'' gene complex in SCC''mec'', comprising ''mec'' gene, its regulators (''mecR1'', ''mecI''), and insertion sequences (IS), is categorized into five classes (A to E). Class A includes ''mecA'', full ''mecR1'', ''mecI'', and IS''431''. Class B has IS''1272'', ''mecA'', partial ''mecR1'', and IS''431''. Class C, with two versions (C1, C2), contains ''mecA'', partial ''mecR1'', IS''431'', differing in IS''431'' orientation. Class D includes IS''431'', ''mecA'', partial ''mecR1''; Class E consists of ''blaZ'', ''mecC'', ''mecR1'', ''mecI''. The ''ccr'' and ''mec'' gene complexes in SCC''mec'' are connected by joining (J) regions, considered non-essential but capable of carrying extra antimicrobial resistance determinants. These are categorized as J1, J2, and J3, based on their SCC''mec'' positions. J1, also known as the L-C region, lies between the right chromosomal junction and upstream of the ''ccr'' gene. J2, previously the C-M region, is situated between the ''ccr'' and ''mec'' gene complexes. J3 (formerly the I-R region) is found downstream of the mec gene complex, extending to the left chromosomal junction.

Distribution

The SCC''mec'' found in methicillin-resistant ''Staphylococcus aureus'' likely originated in

coagulase Coagulase is a protein enzyme produced by several microorganisms that enables the conversion of fibrinogen to fibrin. In the laboratory, it is used to distinguish between different types of ''Staphylococcus'' isolates. Importantly, '' S. aureus' ...

-negative staphylococcal species and was acquired by ''S. aureus''. Staphylococcal strains isolated from pig farms were found to carry several different types of SCC''mec'', suggesting that they may serve as a reservoir of these elements.

References

Molecular biology Mobile genetic elements {{genetics-stub

Classification

Distribution

See also

References