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Describe MRSA with its mechanism of resistance, detection, treatment and control.

Microbiology · 2nd Professional — Para-Clinical

Mechanism of Methicillin Resistance

MRSA is Staphylococcus aureus resistant to methicillin and therefore to all beta-lactam antibiotics, through acquisition of the mecA gene.

The mecA gene, carried on a mobile element (SCCmec), encodes an altered penicillin-binding protein, PBP2a → PBP2a has very low affinity for beta-lactams → Cell wall cross-linking therefore continues even when all the normal penicillin-binding proteins are blocked → Resistance extends to every beta-lactam — penicillins, cephalosporins (except ceftaroline) and carbapenems → Because the mechanism is an altered target rather than an enzyme, beta-lactamase inhibitors such as clavulanic acid are useless

CLINICAL PEARL

The distinction between altered target and enzymatic resistance decides what will work. Ordinary penicillin resistance in staphylococci is caused by penicillinase, an enzyme — defeated by a stable penicillin such as cloxacillin. MRSA has changed the target itself, so no amount of protecting the drug helps; an entirely different class is required. Two resistance mechanisms, two completely different clinical responses.

Types and Laboratory Detection

Type	Features
Hospital-acquired (HA-MRSA)	Affects patients with healthcare contact — surgery, devices, prolonged admission, previous antibiotics. Typically multidrug resistant beyond the beta-lactams
Community-acquired (CA-MRSA)	Affects previously healthy people with no healthcare contact; carries PANTON-valentine leucocidin and causes severe necrotising skin infection and necrotising pneumonia. Usually susceptible to non-beta-lactam agents such as clindamycin and co-trimoxazole
Livestock-associated	Related to intensive animal farming and antibiotic use in agriculture
Detection	Cefoxitin disc diffusion is the recommended screening method (a better inducer of mecA expression than oxacillin); chromogenic agar; and PCR for mecA or latex agglutination for PBP2a as confirmation
VISA and VRSA	Vancomycin-intermediate strains have thickened cell walls that trap the drug; fully vancomycin-resistant strains have acquired vanA from enterococci — still rare but of great concern

Treatment

Vancomycin — the standard parenteral agent; trough levels are monitored, and under-dosing causes both failure and resistance
Teicoplanin — similar, with a longer half-life allowing once-daily and intramuscular use
Linezolid — excellent oral bioavailability, so serious infection can be treated orally; causes reversible myelosuppression beyond two weeks, and optic and peripheral neuropathy
Daptomycin — effective in bacteraemia and right-sided endocarditis, but inactivated BY pulmonary surfactant, so it must never be used for pneumonia; monitor creatine kinase
Clindamycin, CO-trimoxazole, doxycycline — useful oral options in community-acquired MRSA skin infection; the D-test must be performed before using clindamycin, to exclude inducible resistance
Ceftaroline — the only beta-lactam with MRSA activity, since it binds PBP2a; mupirocin topically for nasal decolonisation
Combination therapy with rifampicin is used in prosthetic device infection, where rifampicin penetrates biofilm, but it must never be given alone because resistance emerges rapidly

Control and Applied Aspects

Hand hygiene is the single most effective control measure; the hands of staff are the principal vehicle of transmission between patients
Screening and decolonisation of carriers before elective surgery — nasal mupirocin with chlorhexidine body washes — measurably reduces surgical site infection
Contact precautions and cohorting of colonised patients, with environmental cleaning
Antimicrobial stewardship — particularly restricting cephalosporins and fluoroquinolones, which select strongly for MRSA
Remove infected devices; biofilm on a line, prosthesis or valve cannot be sterilised by any antibiotic, and retention causes relapse
MRSA rates in Indian hospitals are high, and infection control discipline matters more than the availability of any particular drug; the newer agents are expensive and their unnecessary use will produce resistance in turn

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