Characterization of SCCmec Instability in Methicillin-Resistant Staphylococcus aureus Affecting Adjacent Chromosomal...

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• Staphylococcal cassette chromosome mec (SCCmec) is crucial in staphylococci, containing the mecA gene for methicillin resistance.
• SCCmec elements can be unstable, with internal mobile elements and recombinases causing rearrangements.
• SCCmec alterations may impact adjacent chromosomal sequences, such as the staphylococcal protein A (spa) gene.

Purpose of the Study:

• To investigate the stability of SCCmec in methicillin-resistant Staphylococcus aureus (MRSA) strains.
• To determine the impact of culturing without antimicrobial selection on SCCmec integrity.
• To identify factors influencing SCCmec stability, including mobile elements and host cell environment.

Main Methods:

• MRSA strains with four different SCCmec types were cultured without antimicrobial selection for 3 months.
• Cefoxitin susceptibility was monitored as an indicator of SCCmec rearrangements.
• Sequence analysis was performed to characterize SCCmec alterations and adjacent chromosomal changes.

Main Results:

• SCCmec rearrangements were detected in cefoxitin-susceptible variants after 2 months.
• The most common rearrangement was precise excision of the entire SCCmec element.
• Internal SCCmec deletions, some involving loss of the spa gene, were observed after 3 months.
• No loss of only the mecA gene (empty cassettes) was detected.

Conclusions:

• SCCmec stability is influenced by internal mobile elements like IS431 and the host cell environment.
• SCCmec rearrangements, including precise excision and deletions, can occur even without antibiotic pressure.
• These findings highlight the dynamic nature of SCCmec and its potential impact on the staphylococcal genome.