Gramicidin S (GrS)

Gramicidin S is an antibiotic effective against some Gram positive and Gram negative bacteria as well as some fungi. It is a derivative of gramicidin, produced by the Gram positive bacterium Bacillus brevis. Gramicidin S is a cyclodecapeptide, It forms a ring structure composed of five different amino acids, each one used twice within the structure.Another interesting point is that it utilizes two amino acids uncommon in peptides: ornithine as well as the unnatural stereoisomer of phenylalanine. It is synthesized by gramicidin S synthetase.Structurally, Gramicidin S differs from Gramicidin D, which is a linear peptide and forms a beta helix in cellular membranes. The mode of action is not entirely agreed upon, but it is generally accepted that it is the disruption of the barrier properties of cellular membranes which causes cell death.