MRSA.

MRSA Definition:

 Methicillin, MIC 8 μg/ml or more, Oxacillin MIC 4 μg/ml or more.

Resistance is mediated by mecA gene which is chromosomally coded.

It alters PBP (penicillin binding protein)present on s.aureus cell membrane to PBP-2a which has less affinity for beta-lactam antibiotics.

Penicillin Binding Protein :Normal protein present in cell membrane.

Transpeptidase in nature.

Required for cross-linking of the peptidoglycan layer (essential for cell wall synthesis).

It is also the site of attachment for- beta-lactam drugs to bind & inhibit.

Types of MRSA:

Community-acquired - mediated by mec A subtype IV.

Possess- PV Toxin(Panton-Valentine Toxin), multidrug resistant.

Hospital-acquired - mediated by mec A subtype I, II, III, mono-drug resistance

Detection of MRSA:

Antibiotic susceptibility test on Muller Hilton agar (Disk diffusion test) by using Methicillin or Oxacillin disc or cefoxitin disc.

PCR for detection of mecA gene.

Conditions to be maintained for Incubation:

On media containing 4% NaCl

Incubation at 30°C

Full 24hr incubation

Treatment:

Vancomycin is DOC. (should be used only if other drugs are resistant ).

Newer Drugs – Daptomycin, ceftobiprole, tigecycline, oritavancin.

Ceftobiprole is the only β lactameffective against MRSA.