Gramicidin

Gramicidin refers to a group of antimicrobial peptides originally isolated from the soil bacterium Bacillus brevis (now classified as Brevibacillus brevis) in 1939 by René Dubos. It was one of the first antibiotics discovered and used clinically. The term gramicidin encompasses two main types: the linear gramicidins (A, B, and C, collectively called gramicidin D) and gramicidin S, a cyclic decapeptide. Linear gramicidin A is the most abundant and clinically used form.

Linear gramicidin's antimicrobial mechanism involves the formation of ion channels in bacterial cell membranes. Two gramicidin molecules dimerize in a head-to-head fashion within the lipid bilayer, forming a transmembrane pore that allows monovalent cations (sodium, potassium, hydrogen) to flow freely across the membrane. This uncontrolled ion flux disrupts the bacterial cell's electrochemical gradient and energy metabolism, ultimately causing cell death. This channel-forming activity also makes gramicidin one of the most studied peptides in biophysics and membrane biology.

Gramicidin S has a different structure and mechanism. As a cyclic decapeptide containing two D-amino acids, it acts as a surface-active agent that disrupts membrane integrity through a detergent-like mechanism rather than channel formation. Both forms are effective against Gram-positive bacteria, with gramicidin S also showing some activity against Gram-negative organisms.

Due to its hemolytic activity and toxicity to mammalian cells, gramicidin is restricted to topical use. It is most commonly found in ophthalmic preparations, often combined with neomycin and polymyxin B, and in some topical skin infection treatments. Despite its toxicity limitations for systemic use, gramicidin remains an important tool in both clinical medicine and scientific research as a model ion channel peptide.