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.