Gramicidin is a heterogeneous mixture of three antibiotic compounds,
gramicidins A,
B and
C, making up 80%, 6%, and 14%, respectively, all of which are obtained from the soil bacterial species
Bacillus brevis and called collectively
gramicidin D. Gramicidin D contains
linear pentadecapeptides, that is chains made up of 15
amino acid. This is in contrast to
gramicidin S, which is a cyclic peptide chain.