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Updated: Feb 2, 2026
Multiplex PCR Assay for Typing of Staphylococcal Cassette Chromosome Mec Types I to V in Methicillin-resistant Staphylococcus aureus
Published on: September 5, 2013
Characterization of Staphylococcal Cassette Chromosome mec Elements from Methicillin-Resistant Staphylococcus
Kate A Worthing1,2, Sybille Schwendener3, Vincent Perreten3
1Sydney School of Veterinary Science, University of Sydney, NSW, Australia.
Abstract:
We examined the oxacillin resistance phenotype and genomic structure of staphylococcal cassette chromosome mec (SCCmec) elements from 77 veterinary methicillin-resistant Staphylococcus pseudintermedius (MRSP) isolates. Isolates were characterized by oxacillin broth microdilution, whole-genome sequencing, and bioformatics analysis. Five previously described SCCmec elements, and a sixth novel element, were identified: SCCmec III (also known as II-III), ΨSCCmec57395, and SCCmecNA45 (a SCCmec VII variant), all previously described in MRSP, and SCCmec IVg and SCCmec VT, previously described in both methicillin-resistant Staphylococcus aureus (MRSA) and MRSP. The sixth element was novel and found among nine geographically clustered isolates. This novel pseudostaphylococcal cassette chromosome (ΨSCCmecKW21) contained a class A mec gene complex but lacked ccr genes. It also harbored heavy metal (cadmium) resistance determinants. The median oxacillin MIC values among ΨSCCmecKW21, SCCmec III, and SCCmec VT isolates were significantly higher than those determined for the SCCmecNA45 VII variant isolates and ΨSCCmec57395 and SCCmec IVg isolates. ΨSCCmecKW21 was found exclusively in sequence type 497 (ST497), an MRSP clone that is locally successful in Victoria, Australia. Future studies are necessary to determine if this clone has disseminated further afield and if ΨSCCmecKW21 has moved into other MRSP lineages or staphylococcal species.IMPORTANCEStaphylococcus pseudintermedius is a significant veterinary pathogen and occasional cause of infections in humans. β-Lactams are an important group of antimicrobials used to treat staphylococcal infections in humans and animals. However, when staphylococci become methicillin resistant via the acquisition of a mobile genetic element called staphylococcal cassette chromosome mec (SCCmec), they become resistant to all β-lactams. This study detected a novel SCCmec element among a cluster of methicillin-resistant S. pseudintermedius isolates from animals in Australia. It also detected SCCmec elements in S. pseudintermedius that had high similarity to those identified in methicillin-resistant Staphylococcus aureus, demonstrating how human and animal pathogens can share the same resistance determinants.
Insights
A novel staphylococcal cassette chromosome mec (SCCmec) element, ΨSCCmecKW21, was identified in veterinary methicillin-resistant Staphylococcus pseudintermedius (MRSP) from Australia. This element is associated with higher oxacillin resistance and heavy metal resistance, highlighting shared resistance determinants between animal and human pathogens.
Area of Science:
- Veterinary Microbiology
- Antimicrobial Resistance
- Genomics
Background:
- Staphylococcus pseudintermedius is a key veterinary pathogen and an opportunistic human pathogen.
- Methicillin resistance in staphylococci, mediated by staphylococcal cassette chromosome mec (SCCmec) elements, confers resistance to all beta-lactam antimicrobials.
- Understanding the diversity and evolution of SCCmec elements is crucial for combating antimicrobial resistance in both animal and human health.
Purpose of the Study:
- To characterize the oxacillin resistance phenotype and genomic structure of SCCmec elements in veterinary MRSP isolates.
- To identify and describe novel SCCmec elements and their associated genetic features.
- To investigate the genetic relatedness and geographical distribution of MRSP clones carrying specific SCCmec elements.
Main Methods:
- Oxacillin broth microdilution for determining resistance phenotypes.
- Whole-genome sequencing for analyzing the genomic structure of SCCmec elements.
- Bioinformatic analysis for identifying known and novel SCCmec elements and characterizing MRSP isolates.
Main Results:
- Six SCCmec elements were identified, including five previously described and one novel element, ΨSCCmecKW21.
- The novel ΨSCCmecKW21 element, found in a geographically clustered MRSP ST497 clone in Australia, contained a class A mec gene complex, lacked ccr genes, and harbored cadmium resistance determinants.
- Isolates with ΨSCCmecKW21, SCCmec III, and SCCmec VT exhibited significantly higher median oxacillin MIC values compared to isolates with other SCCmec types.
Conclusions:
- A novel SCCmec element, ΨSCCmecKW21, associated with high-level oxacillin resistance and heavy metal resistance, was discovered in Australian MRSP.
- The study highlights the potential for SCCmec elements and resistance determinants to be shared between MRSA and MRSP, emphasizing the One Health approach to antimicrobial resistance.
- Further research is needed to determine the dissemination of the ST497 clone and the ΨSCCmecKW21 element into other staphylococcal lineages or species.
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