Linezolid
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Direct answer
Linezolid is the first oxazolidinone antibiotic and works by a unique mechanism: it binds the 50S ribosomal subunit at the peptidyl site and prevents formation of the 70S initiation complex, blocking protein synthesis before the chain begins. It has complete oral bioavailability and reliable activity against resistant gram-positive pathogens — MRSA, vancomycin-resistant enterococci and penicillin-resistant pneumococci. Duration beyond two weeks brings thrombocytopenia, neuropathy and lactic acidosis.
What you must remember
- Mechanism: prevents assembly of the initiation complex at the bacterial 70S ribosome, a step untouched by other classes — hence no cross-resistance with macrolides, aminoglycosides or vancomycin.
- Bacteriostatic against staphylococci and enterococci, an important limitation in bacteraemic endocarditis-type infection where bactericidal agents are preferred.
- Pharmacokinetics: one hundred per cent oral bioavailability with equivalent intravenous dosing, wide tissue distribution including lung and cerebrospinal fluid, and non-renal hepatic metabolism — no dose adjustment in renal failure.
- Key indications: vancomycin-resistant Enterococcus faecium infection, MRSA pneumonia and complicated skin and soft-tissue infection, off-label synergy regimens for mycobacterial and NDM-gram-positive mixed infection, and second-line therapy in drug-resistant tuberculosis.
- Time-dependent toxicity: thrombocytopenia and anaemia after more than ten to fourteen days of therapy, monitoring blood counts weekly in prolonged use.
- Linezolid is a weak monoamine oxidase inhibitor — serotonin syndrome can occur with selective serotonin reuptake inhibitors, and blood pressure rises with sympathomimetics or tyramine-rich food.
- Prolonged therapy risks peripheral and optic neuropathy that may be only partially reversible, plus lactic acidosis from mitochondrial toxicity; yearly fundus review is prudent on chronic therapy.
Common confusion
Students bracket linezolid with vancomycin as interchangeable MRSA drugs. Vancomycin is bactericidal, renal-cleared, level-monitored and intravenous; linezolid is bacteriostatic, orally bioavailable, freely usable in renal failure and limited by duration-dependent marrow and nerve toxicity. Examiners reward precisely this contrast, and also the distinction that red man syndrome belongs to vancomycin while serotonin syndrome belongs to linezolid.
Exam-focused takeaway
Theory answers should highlight the unique initiation-complex mechanism, oral bioavailability, resistant gram-positive niche and duration-limited toxicities. Viva examiners ask why linezolid suits renal failure and outpatient VRE therapy but not endocarditis. MCQs test the 50S initiation site, thrombocytopenia beyond two weeks, serotonin syndrome with SSRIs, optic neuropathy and the absence of cross-resistance.
Frequently asked questions
How does linezolid differ mechanistically from other 50S inhibitors?
It blocks formation of the 70S initiation complex rather than elongation or translocation, so cross-resistance with macrolides, chloramphenicol or clindamycin does not occur.
Which patients are the prime candidates for linezolid?
Those with MRSA pneumonia or skin infection, vancomycin-resistant enterococcal infection, or significant renal impairment where vancomycin dosing is difficult, provided bactericidal therapy is not mandatory.
What monitoring is needed during prolonged linezolid therapy?
Weekly complete blood counts for thrombocytopenia and anaemia, review for peripheral neuropathy, visual acuity and colour vision for optic neuropathy.
Why can linezolid trigger serotonin syndrome?
It weakly inhibits monoamine oxidase, so concurrent selective serotonin reuptake inhibitors or serotonergic tramadol can raise synaptic serotonin dangerously.
Is linezolid bactericidal or bacteriostatic?
Bacteriostatic against staphylococci and enterococci, which is why it is avoided for enterococcal endocarditis in favour of synergistic bactericidal combinations.