Gramicidin

Component of Neosporin/Polysporin eye drops (topical)

Gramicidin holds a special place in the history of medicine: isolated in the late 1930s from the soil bacterium Bacillus brevis (as part of the mixture tyrothricin), it was among the first antibiotics to be produced and used clinically, predating the widespread availability of penicillin. Its mechanism is unusual and elegant. Gramicidin is a short, hydrophobic linear peptide that inserts into lipid membranes and — by pairing head-to-head with another gramicidin molecule — forms a transmembrane channel. That channel is selectively permeable to small monovalent cations such as sodium and potassium. By opening an uncontrolled leak for these ions, gramicidin collapses the electrochemical gradients a bacterium needs to survive, killing Gram-positive organisms. Because gramicidin also disrupts mammalian cell membranes and can rupture red blood cells (hemolysis) if it reaches the bloodstream, it is never given systemically. Instead it is used topically: it appears in some over-the-counter and prescription eye drops and topical antibiotic combinations (for example alongside neomycin and polymyxin B), where it contributes Gram-positive coverage at the surface without systemic exposure. Beyond the clinic, gramicidin is one of the most studied molecules in all of membrane biophysics. Its well-defined, structurally simple ion channel has served for decades as a model system for understanding how peptides fold in membranes and how ion channels conduct — foundational knowledge that informs peptide and drug design broadly. Gramicidin is an approved ingredient in topical products; it is a historical and scientific landmark as much as a working antibiotic, and it is not a systemic therapy or a research-injection peptide.