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How does methicillin-resistant Staphylococcus aureus (MRSA) differ from methicillin-sensitive?

The DSI Report

Methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA) are two forms of the bacterium Staphylococcus aureus that differ in their susceptibility to antibiotics, particularly methicillin and other beta-lactam antibiotics. Here are the key differences between MRSA and MSSA:

Antibiotic Resistance: 


Methicillin-resistant Staphylococcus aureus is resistant to methicillin and other beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems. MRSA strains have acquired a mecA or mecC gene, which encodes an altered penicillin-binding protein (PBP2a or PBP2c) with reduced affinity for beta-lactam antibiotics, rendering them ineffective.


Methicillin-sensitive Staphylococcus aureus is susceptible to methicillin and other beta-lactam antibiotics. MSSA strains lack the mecA or mecC gene and are typically sensitive to penicillins, cephalosporins, and other beta-lactam antibiotics.



MRSA strains were historically associated with healthcare-associated infections (HA-MRSA), occurring in hospitals, long-term care facilities, and other healthcare settings. However, community-associated MRSA (CA-MRSA) strains have emerged and can cause infections in otherwise healthy individuals outside of healthcare settings, such as in the community, schools, sports teams, and correctional facilities.


MSSA infections can occur in both healthcare and community settings but are typically more common in the community. MSSA strains are often implicated in skin and soft tissue infections, as well as respiratory and bloodstream infections.

Treatment Options:

MRSA: Treatment of MRSA infections can be challenging due to antibiotic resistance. MRSA infections may require treatment with alternative antibiotics, such as vancomycin, daptomycin, linezolid, or newer agents like ceftaroline or dalbavancin. Some MRSA strains may also exhibit resistance to non-beta-lactam antibiotics, limiting treatment options.


MSSA infections are generally treatable with beta-lactam antibiotics, such as penicillins (e.g., oxacillin, nafcillin) or cephalosporins (e.g., cefazolin). These antibiotics are typically effective against MSSA strains, although resistance can occur in rare cases.

Virulence Factors:

MRSA: While MRSA strains share many virulence factors with MSSA, some studies suggest that certain MRSA strains may possess additional virulence factors or exhibit enhanced pathogenicity compared to MSSA strains.      


MSSA strains can produce a variety of virulence factors, including toxins, enzymes, and adhesins, which contribute to their ability to cause disease. However, the virulence profiles of MSSA strains may vary depending on the strain type and genetic background.

Overall, the emergence of MRSA has posed significant challenges for infection control and treatment, particularly in healthcare settings. Understanding the differences between MRSA and MSSA is essential for guiding appropriate antibiotic therapy and implementing effective strategies to prevent the spread of antibiotic-resistant bacteria.

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