Methicillin-Resistant Staphylococcus Aureus Infection (MRSA)

Colonization versus infection - A positive nasal or skin swab tells you the organism is part of that patient's flora, not that they are infected. Culture from a sterile site alongside clinical signs makes the diagnosis; a swab alone does not. Treat the isolation status as an infection-control fact and the culture as the clinical one.

Know who is likely to be colonized - Health care workers, people who use needles regularly (diabetics, IV drug users), hospitalized patients, and immunocompromised patients carry S. aureus at rates up to 80%. Documented MRSA risk factors are age over 65, an indwelling urinary catheter, antibiotics within the past three months, trauma, and admission from a long-term care facility.

Decolonization before surgery - Topical mupirocin can eliminate nasal carriage in some carriers, though the practice remains debated. If the surgical service ordered a decolonization protocol, find out whether it was actually completed, because it feeds directly into the prophylaxis decision.

Prophylaxis changes when MRSA is the concern - Cefazolin does not cover MRSA; vancomycin is the standard agent for MRSA strains, with trimethoprim/sulfamethoxazole, clindamycin, and doxycycline used orally for mild to moderate disease and linezolid, daptomycin, or tigecycline as alternatives in severe disease. The substitution to vancomycin is the one with intraoperative consequences, because the drug has to go in slowly.

Vancomycin flushing syndrome is rate-dependent, not allergic - Rapid infusion causes direct, non-immune histamine release from mast cells and basophils. Give no faster than 10 mg/min or 1 g/hour. It typically begins 4 to 10 minutes after the first dose starts, but can appear later in the infusion, after the dose finishes, or on doses given up to seven days out.

Recognize it in a patient who cannot tell you - Awake patients report pruritus and show erythema over the face, neck, and upper torso. Under anesthesia you get the rest of the syndrome instead: hypotension, tachycardia, angioedema, muscle spasm, and chest or back pain. It occurs in 4% to 50% of treated patients and is most severe under age 40.

Manage it - Stop the infusion. Diphenhydramine 50 mg IV and cimetidine 300 mg IV, with normal saline boluses for the hypotension. Most episodes settle within 20 minutes and vancomycin can be restarted at half the previous rate. If the drug must continue, premedicate with the same two agents an hour before and run the dose over four hours.

Separate it from anaphylaxis - Both give pruritus, erythematous rash, and tachycardia. Stridor, hives, angioedema, and wheeze point toward IgE-mediated anaphylaxis, which requires prior exposure; flushing syndrome is rate-related and usually occurs on first exposure. If the picture includes altered mental status, stridor, or bronchospasm, treat it as anaphylaxis and give epinephrine early rather than debating the mechanism.

Protect the kidneys and the ears - Vancomycin nephrotoxicity rises with exposure, which is why dosing targets an AUC/MIC ratio and is adjusted in altered kidney function rather than given as a fixed dose. Ototoxicity — tinnitus, sensorineural hearing loss, vertigo — is less common but can be irreversible. Look at the creatinine, and think twice before stacking another nephrotoxin on the same day.

Linezolid is a reversible, non-selective MAO inhibitor - If the MRSA regimen includes linezolid, monoamine oxidase inhibition raises epinephrine, norepinephrine, dopamine, and serotonin centrally and in the sympathetic nervous system. Adrenergic drugs can produce exaggerated hypertension, and serotonergic agents raise the risk of serotonin syndrome. Start vasopressors low and titrate, and re-examine the serotonergic pieces of your multimodal plan.

Prosthetic and device infections are a different operation - MRSA on a prosthesis or a catheter usually means removal of the hardware, not just antibiotics, because biofilm protects the organism. Expect a longer, dirtier case with revision anatomy rather than a straightforward washout.

Staphylococcus aureus is a gram-positive coccus that lives as normal flora on skin and mucous membranes — roughly half of adults are colonized and about 15% carry it persistently in the anterior nares. Methicillin-resistant strains carry the mec gene within the SCCmec cassette, which encodes penicillin-binding protein 2a. PBP-2a has low binding affinity for beta-lactams, so the organism keeps building its cell wall in the presence of methicillin, nafcillin, oxacillin, and the cephalosporins.

Colonization is not infection. S. aureus does not cause disease on intact skin; it causes disease when it crosses into tissue or bloodstream through a break in the skin, an indwelling line, or a surgical wound, and then produces bacteremia, endocarditis, skin and soft tissue infection, osteomyelitis, prosthetic device infection, necrotizing pneumonia, and toxic shock — using capsule, protein A, and biofilm to evade the host response.