Gram­i­cidin is a pep­tide antibi­ot­ic pro­duced by cer­tain strains of soil bac­te­ria (Bre­vibacil­lus bre­vis). It works by form­ing a cation-selec­tive chan­nel in the cyto­plas­mic mem­brane of tar­get bac­te­ria, lead­ing to cell death. Gram­i­cidin is effec­tive against a wide range of Gram-pos­i­tive bac­te­ria but not against Gram-neg­a­tive due to the pres­ence of an out­er mem­brane. The dis­cov­ery of gram­i­cidin was made in 1939 by René Dubos while work­ing at the Rock­e­feller Insti­tute for Med­ical Research in New York.