Spontaneous staphylococcal cassette chromosome mec element excision in Staphylococcus aureus nasal carriers

Area of Science:

• Microbiology
• Infectious Diseases
• Genetics

Background:

• Methicillin-resistant Staphylococcus aureus (MRSA) is a significant public health concern.
• Understanding the dynamics of MRSA and methicillin-susceptible S. aureus (MSSA) co-colonization is crucial for infection control.

Purpose of the Study:

• To investigate the prevalence and genetic relationship of MSSA in patients already colonized with MRSA.
• To explore the potential for in vivo genetic exchange or modification between MRSA and MSSA strains.

Main Methods:

• Pulsed-field gel electrophoresis (PFGE) for DNA fingerprinting.
• Multilocus sequence typing (MLST) for bacterial strain characterization.
• Staphylococcal protein A (spa) typing for genotyping.

Main Results:

• Out of 23 patients with MRSA nasal colonization, 6 (26%) also carried MSSA.
• In 4 of these 6 patients, MSSA strains were genetically distinct from MRSA strains.
• In 2 patients, MSSA strains were identical to MRSA strains except for the absence of the staphylococcal cassette chromosome mec (SCCmec) element.

Conclusions:

• The findings suggest that MSSA can co-exist with MRSA in nasal carriage.
• Evidence points towards spontaneous excision of the SCCmec element in vivo, leading to the emergence of MSSA from MRSA strains.
• This highlights a potential mechanism for MRSA to revert to a susceptible state during colonization.