Methicillin Resistant Staphylococcus aureus (MRSA)

MRSA is mediated by the mecA gene; which is chromosomally coded. It alters penicillin-binding protein (PBP) present on S. aureus cell membrane to PBP-2a:

  • PBP is an essential protein needed for the cell wall synthesis of bacteria. β lactam drugs bind and inhibit this protein, thereby inhibiting cell wall synthesis.
  • The altered PBP2a of MRSA strains has less affinity for β lactam antibiotics; hence MRSA strains are resistant to all β lactam antibiotics.
  • BORSA strains (Borderline Oxacillin resistant S. aureus): Occasionally a non-mecA gene-mediated low-level resistance to oxacillin is observed in some strains of S.aureus, which is due to hyperproduction of β lactamase.
  • There is an increasing trend in the MRSA rates over the last few decades. Though it varies from place to place, overall about 30–40% of strains of S. aureus are MRSA.
  • MRSA rate in India is 30-40%. It is lowest in Scandinavian countries.

Methicillin Resistant Staphylococcus aureus (MRSA)

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Types of MRSA

MRSA are either community or hospital-associated.

Community-associated MRSA (CA-MRSA)

  • It is mediated by the mecA gene subtype IV, V, VI.
  • They are usually more virulent and express several toxins such as PV toxin.
  • They cause invasive skin and soft tissue infections such as necrotizing fasciitis.

Hospital associated MRSA (HA-MRSA)

  • It is mediated by the mecA gene subtype I, II, III.
  • They are multidrug-resistant (but their virulence is low).
  • They cause perioperative wound infections in hospitals and nosocomial outbreaks (hospital staff are the major carries).

Note: CA-MRSA and HA-MRSA terminologies are becoming artificial nowadays; as many CA-MRSA strains have been isolated in hospitals and vice versa.

Detection of MRSA

  • Antimicrobial susceptibility test: Disk diffusion test can be done by using cefoxitin or oxacillin disks.
    • Cefoxitin is the recommended disk to be used.
    • If an oxacillin disk is used, then certain conditions to be maintained such as—using media containing 2–4% NaCl, incubation at 30 °C for 24 hours.
  • Oxacillin screen agar: Adding oxacillin 6 μg/ml and NaCl (2–4%) to the medium.
  • PCR detecting mecA gene.
  • Latex agglutination test detecting PBP-2a

Treatment of MRSA

  • Vancomycin is the drug of choice for MRSA.
  • Alternate drugs include:
    • Teicoplanin, linezolid, quinupristin-dalfopristin, tigecycline, oritavancin.
    • Daptomycin (for endocarditis and complicated skin infections),
    • Mupirocin 2% ointment (for nasal carriers of MRSA).
  • However, even simple orally effective drugs such as tetracycline, erythromycin, or cotrimoxazole may also be effective. These can be indicated in non-serious conditions, caused by CA-MRSA strains if found to be susceptible based on antimicrobial susceptibility report.
  • All β lactam drugs should be avoided. However, 5th generation cephalosporins, such as Ceftobiprole, ceftaroline, ceftolozane have shown some activity against MRSA.


  1. Apurba Sankar Sastry and Sandhya Bhat K (2018). Review of Microbiology and Immunology. Sixth Edition. Jaypee Brothers Medical Publishers (P) Ltd. ISBN: 978-93-86322-39-5.

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

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