We have isolated from the hemolymph of immunized larvae of the dipteran insect Phormia terranovae two peptides that are selectively active against Gram-positive bacteria. They are positively charged peptides of 40 residues containing three intramolecular disulfide bridges and differ from one another by only a single amino acid. These peptides are neither functionally nor structurally related to any known insect immune response peptides but show significant homology to microbicidal cationic peptides from mammalian granulocytes (defensins). We propose the name "insect defensins" for these insect antibiotic peptides.

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Diptericin A is a member o f a multigenic family of antibacterial peptides that are synthesized by larvae of fhormia terranovae (Diptera) in response to a bacterial injection o r to injury. The 82-residue peptide is active only against a limited range o f Gram-negative bacteria. Data presented suggest that the primary action of diptericin A is on the cytoplasmic membrane of growing bacteria.