Aminoglycosides
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Direct answer
Aminoglycosides — gentamicin, amikacin, streptomycin, tobramycin and neomycin — are bactericidal inhibitors of bacterial protein synthesis that bind the 30S ribosomal subunit and cause misreading of messenger RNA. They are reserved for serious aerobic gram-negative infections and, in synergy with beta-lactams or vancomycin, for enterococcal endocarditis. Their uptake into bacteria is oxygen-dependent, so they fail against anaerobes, and their use is capped by nephrotoxicity and ototoxicity.
What you must remember
- Mechanism: irreversible binding to the 30S subunit prevents the initiation complex and distorts codon-anticodon pairing; transport in is an oxygen-dependent active process — hence no activity against anaerobes or in acidic, hypoxic abscess cavities.
- Pharmacology: negligibly absorbed orally (oral neomycin was used to reduce ammonia-producing gut flora), confined largely to extracellular fluid with poor cerebrospinal penetration, and excreted unchanged by glomerular filtration — dose reduction is mandatory in renal impairment.
- They exhibit concentration-dependent killing plus a substantial post-antibiotic effect, which underpins once-daily dosing for most indications; divided dosing remains traditional for enterococcal endocarditis synergy.
- Monitoring: trough levels (with peaks where indicated), serial serum creatinine, and enquiry about tinnitus, hearing loss or unsteadiness.
- Nephrotoxicity is usually reversible and targets the proximal tubule; risk rises with volume depletion and with concomitant vancomycin, amphotericin B or loop diuretics.
- Ototoxicity is often irreversible, may be cochlear (deafness) or vestibular (vertigo, ataxia), is potentiated by loop diuretics, and streptomycin is avoided in pregnancy for fetal ear damage.
- Rapid intravenous injection can provoke neuromuscular blockade with respiratory weakness — worse in myasthenia gravis and hypocalcaemia, reversed by calcium gluconate with neostigmine.
- Individual agents: amikacin resists most aminoglycoside-inactivating enzymes (used in resistant gram-negatives and multidrug-resistant tuberculosis); tobramycin is antipseudomonal, including inhaled in cystic fibrosis; neomycin is restricted to topical use as the most toxic member.
Common confusion
The classic muddle is explaining why an antibiotic so lethal to gram-negative aerobes is useless against anaerobes — the answer is the oxygen-dependent transport that carries it across the bacterial membrane, not the ribosomal target. Equally, reversible nephrotoxicity and irreversible ototoxicity are swapped by candidates; fixing "kidneys recover, ears may not" answers both viva and MCQ versions.
Exam-focused takeaway
Theory answers should cover mechanism with the oxygen-dependence caveat, clinical uses, the toxicity pair with its risk factors, and monitoring. Viva examiners ask about the post-antibiotic effect, once-daily dosing logic and enterococcal synergy. MCQs test the 30S target, inactivity against anaerobes, the calcium-responsive neuromuscular blockade, streptomycin in tuberculosis and neomycin as a topical-only drug.
Frequently asked questions
Why are aminoglycosides inactive against anaerobes?
Their uptake requires oxygen-dependent active transport across the bacterial membrane, which obligate anaerobes cannot perform.
Which aminoglycoside toxicity is reversible and which is not?
Nephrotoxicity is usually reversible on stopping the drug; ototoxicity — cochlear or vestibular — is frequently irreversible.
What is the post-antibiotic effect?
Persistent suppression of bacterial growth after antibiotic concentrations fall below the minimum inhibitory concentration, allowing effective once-daily dosing.
Why combine an aminoglycoside with a beta-lactam in enterococcal endocarditis?
The cell-wall agent damages the membrane and enhances aminoglycoside entry, producing bactericidal synergy inside the vegetations.
How is aminoglycoside neuromuscular blockade managed?
With intravenous calcium gluconate and neostigmine; the reaction is a reminder to avoid aminoglycosides in myasthenia gravis.