Introduction

Methicillin resistant Staphylococcal aureus (MRSA) is a major pathogen of nosocomial and community acquired infections associating with significant morbidity and mortality rates of 15-60% [1]. Resistance to methicillin is conferred by the expression of PBP 2a protein which is encoded by the methicillin resistance gene, mecA gene, which is situated on a unique mobile genetic element known as the “staphylococcal cassette chromosome mec” (SCCmec) [2] .

The emergence of MRSA from methicillin susceptible S. aureus (MSSA) is due to site-specific integration of SCCmec into the orfX locus in the chromosome of a susceptible isolate. This type of resistance was termed “intrinsic resistance” because it was not due to destruction of the antibiotic by β-lactamase [3].

SCCmec elements are highly diverse in their structural organization and genetic content and have been classified into types and subtypes. SCCmec consists of three regions: a mec complex carrying mecA and, if present, its regulatory genes mecI and mecR1, a ccr complex carrying cassette chromosome recombinase (ccr) genes which encodes site specific recombinases, namely ccrA/B that mediate the integration of SCCmec into and its excision from the recipient chromosome and are, therefore, responsible for its mobility and a series of variable “junkyard” or J regions (J1, J2 and J3), which constitute nonessential components of SCCmec; although, in some cases, these regions carry additional antibiotic resistance determinants [4]. SCCmec elements can be typed by first identifying the mec and ccr type, and then identifying genes in the J-regions [5].

Significant geographic variations have been found in the structural organization of the SCCmec and these variations have been used to classify the SCCmec types [6]. Following the first report of the element, many structurally distinct SCCmec elements have been identified. To date, nucleotide sequences of more than 80 SCCmec elements have been determined in staphylococcal species. The SCCmec elements identified in S. aureus are classified into types first based on the combination of two essential components, the mec gene complex and the ccr gene complex. Eleven types have been reported so far based on the criteria suggested by IWG-SCC [7], but only type I-V are globally distributed, whilst others appear to exist as local strains in the country of origin [8, 9]. The updated list and the classification of SCCmec are available at https://www.SCCmec.org or https://www.staphylococcus.net. Polymorphism in J regions (mainly J1) have been described and used for the definition of SCCmec type IV subtypes (IVa- IVg) [10, 11].

Finally, the global emergence of MRSA has turned into a serious public health problem. Monitoring and limiting the spread of MRSA strains requires the use of rapid and precise epidemiologic typing systems. MRSA typing is essential for the establishment of national and international surveillance networks. In MRSA typing, both phenotypic and genotypic methods are used. These methods vary greatly in their discriminatory power, reproducibility of the results and the cost and efforts required. Molecular typing is a powerful tool that can provide information about genetic characteristics of a specific clone. A large number of molecular methods have been developed for typing MRSA strains. SCCmec typing is an important molecular tool and it is also of a potential clinical and epidemiologic interest for the analysis of MRSA [12]. Thus, characterization of SCCmec types can be a good marker to reveal the epidemiological change of MRSA infection [13].

References

1. Cosgrove SE, Sakoulas G, Perencevich EN, Schwaber MJ, Karchmer AW, et al. (2003) Comparison of mortality associated with methicillin-resistant and methicillin-susceptible Staphylococcus aureus bacteremia: A meta-analysis. Clin Infect Dis 36: 53-59.
2. Deurenberg RH, Stobberingh EE (2008) The evolution of Staphylococcus aureus. Infect Genet Evol 8: 747-763.
3. Chambers HF (1997) Methicillin resistance in staphylococci: Molecular and Biochemical Basis and Clinical Implications. Clin Microbiol Rev 10: 781-791.
4. Ito T, Okuma K, Ma XX, Yuzawa H, Hiramatsu K (2003) Insights on antibiotic resistance of Staphylococcus aureus from its whole genome: Genomic island SCC. Drug Resist Updat 6: 41-52.
5. Ito T, Kuwahara-Arai K, Katayama Y, Uehara Y, Han X, et al. (2014) Staphylococcal cassette chromosome mec (SCCmec) analysis of MRSA. Methods Mol Biol 1085: 131-148.
6. Turlej A, Hryniewicz W, Empel J (2011) Staphylococcal cassette chromosome mec (SCCmec) classification and typing methods: an overview. Polish Journal of Microbiology 60: 95-103.
7. International Working Group on the Classification of Staphylococcal Cassette Chromosome Elements (IWG-SCC).
8. Oliveira DC, Milheirico C, De Lencastre H (2006) Redefining a structural variant of staphylococcal cassette chromosome mec, SCCmec type VI. Antimicrob Agents Chemother 50: 3457-3459.
9. Zhang K, McClure JA, Elsayed S, Conly JM (2009): Novel staphylococcal cassette chromosome mec type, tentatively designated type VIII, harboring class A mec and type 4 ccr gene complexes in a Canadian epidemic strain of methicillin-resistant Staphylococcus aureus. Antimicrob Agents Chemother 53: 531-540.
10. Pérez-Roth E, Lorenzo-Díaz F, Batista N, Moreno A, Méndez- Alvarez S (2004) Tracking methicillin-resistant Staphylococcus aureus clones during a 5-year period (1998 to 2002) in a Spanish hospital. J Clin Microbiol 42: 4649-4656.
11. Chongtrakool P, Ito T, Ma XX, Kondo Y, Trakulsomboon S, et al. (2006) Staphylococcal cassette chromosome mec (SCCmec) typing of methicillin-resistant Staphylococcus aureus strains isolated in 11 Asian countries: A proposal for a new nomenclature for SCCmec elements. Antimicrob Agents Chemother 50: 1001-1012.
12. McDougal LK, Steward CD, Killgore GE, Chaitram JM, McAllister SK, et al. (2003) Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: Establishing a national database. J Clin Microbiol 41: 5113-5120.
13. Zhang K, McClure JA, Elsayed S, Louie T, Conly JM (2005) Novel multiplex PCR assay for characterization and concomitant subtyping of staphylococcal cassette chromosome mec types I to V in methicillin-resistant Staphylococcus aureus. J Clin Microbiol 43: 5026-5033.