How aminoglycosides are tested on Step 1

Questions ask why an aminoglycoside fails against an abscess or anaerobe, which drug pair is synergistic, or which toxicity explains rising creatinine or hearing loss.

workspace_premium Two facts carry most questions

Aminoglycosides need oxygen to get into bacteria, so they never kill anaerobes. And their toxicity lands on the kidney and the inner ear, with neuromuscular blockade as a third, less common risk.

Mechanism

Aminoglycosides are positively charged and enter gram-negative cells through porins, then cross the cytoplasmic membrane by an oxygen-dependent process.

PorinsCross outermembraneO₂-dependentuptakeFails inanaerobes,acidic pusBind 30SBlockinitiationcomplexMisreadingFaultymembraneproteinsCelldeathBactericidal
Aminoglycoside mechanism. Low pH and low oxygen in abscesses reduce aminoglycoside activity.

Dosing: concentration-dependent killing

Aminoglycosides kill faster at higher peak concentrations and keep suppressing growth after levels fall (the post-antibiotic effect). Toxicity relates to sustained trough levels.

06121824lowmidhighHoursDrug levelOnce dailyEvery 8 hHigh peakLow trough
Once-daily dosing. A high peak maximizes killing; a long drug-free trough lets renal tubular cells and inner-ear hair cells recover, reducing toxicity.

Uses

Aminoglycosides are mainly reserved for serious gram-negative infections and synergy.

AgentMain use
GentamicinGram-negative sepsis; synergy for enterococcal and streptococcal endocarditis
TobramycinPseudomonas, including inhaled in cystic fibrosis
AmikacinResistant gram-negatives; nontuberculous mycobacteria
StreptomycinTuberculosis (second-line), plague, tularemia
NeomycinOral bowel preparation; hepatic encephalopathy (topical use common)

Aminoglycosides penetrate poorly into the CNS and lungs and are inactive in abscesses.

Toxicity

Toxicity depends on cumulative exposure and troughs, and adds up with other nephrotoxic and ototoxic drugs.

Aminoglycoside toxicityNephrotoxicityProximal tubule ATN;usually reversibleOtotoxicityCochlear andvestibular; oftenpermanentNM blockade↓ ACh release; worsein myastheniaTeratogenFetal CN VIII damage
Toxicity. Risk rises with loop diuretics, vancomycin, cisplatin, amphotericin B, and older age; monitor creatinine and drug levels.
functions Worked example — why add gentamicin?

A patient with Enterococcus faecalis endocarditis is treated with ampicillin plus gentamicin. Why is gentamicin added when it does not work on enterococci alone?

  1. Enterococci are relatively impermeable to aminoglycosides.
  2. Ampicillin damages the cell wall, letting gentamicin enter and reach the 30S ribosome.
  3. The combination becomes bactericidal, which is needed in endocarditis.

Answer. Synergy: the cell-wall agent allows aminoglycoside entry. Ceftriaxone plus ampicillin is an alternative that avoids aminoglycoside nephrotoxicity.