Gramicidin S molecule is amphiphilic, with hydrophobic amino acids (D-Phe, Val, Leu side chains) and charged aminoacid (L-Orn). It exhibits strong antibiotic activity towards Gram-negative and Gram-positive and even several pathogenic fungi. The mode of action is not entirely agreed upon, but it is disruption of the lipid membrane and enhancement of the permeability of the bacterial cytoplasmic membrane. Unfortunately, being hemolytic at even low concentrations, gramicidin S is only used as topical applications at present. Additionally, Gramicidin S has been employed as a spermicide and therapeutic for genital ulcers caused by sexually transmitted disease.

Linear gramicidin is a hydrophobic antibiotic polypeptide composed of 15 hydrophobic amino acids in D- and L-alternate conformers with the following sequence: formyl-L-X-gly-L-ala-D-leu-L-ala-D-val-L-val-D-val-L-trp-D-leu-Ltrp-D-leu-L-trp-D-leu-L-trp-ethanolamine where X represents either valine or isoleucine. Whether amino acid 11 is tryptophan, phenylalanine, or tyrosine, gramicidins are called A, B, or C, respectively. A mixture of three pairs of antibacterial substances (Gramicidin A, B, and C) is also called Gramicidin D and is produced by the growth of Bacillus brevis Dubos (Fam. Bacillaceae). Gramicidin is bactericidal for a variety of gram-positive organisms. It increases the permeability of the bacterial cell membrane to inorganic cations by forming a network of channels through the normal lipid bilayer of the membrane. Gramicidin is used in the combination with neomycin sulfate and polymyxin B sulfate (NEOSPORIN®) for the topical treatment of superficial infections of the external eye and its adnexa caused by susceptible bacteria.