Neomycin

Neomycin is an aminoglycoside antibiotic produced by the bacterium Streptomyces fradiae. It exists as a mixture of related compounds, principally neomycin B, neomycin A, and neomycin C. Neomycin inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit, causing misreading of messenger RNA and interference with translation, which can lead to bacterial death.

The antibiotic is active mainly against many Gram-negative enteric bacteria and some Gram-positive cocci; its activity against anaerobes is limited. It is poorly absorbed from the gastrointestinal tract, so oral administration achieves high luminal concentrations with minimal systemic exposure, whereas parenteral use produces significant systemic exposure and a higher risk of toxicity.

Clinical use includes topical application for skin and mucous membrane infections and in ophthalmic preparations; it is commonly formulated in combination products with polymyxin B and bacitracin. Oral or rectal neomycin is used for bowel decontamination before surgical procedures and, in some cases, to reduce intestinal ammonia production in hepatic encephalopathy when used with lactulose.

Safety and adverse effects: systemic exposure carries risks of nephrotoxicity and ototoxicity, especially in patients with renal impairment or concomitant nephrotoxic drugs. Topical and ophthalmic forms can cause contact dermatitis or hypersensitivity reactions. Neomycin should be avoided in individuals with a known hypersensitivity to aminoglycosides and used with caution during pregnancy or in those with neuromuscular disorders such as myasthenia gravis due to potential neuromuscular blockade.

Resistance can arise through aminoglycoside-modifying enzymes and mutations that reduce drug uptake or ribosomal binding, leading to reduced susceptibility and cross-resistance with other aminoglycosides.