An antimicrobial peptide was purified from acidified gill extract of a bivalve mollusk, the American oyster (Crassostrea virginica), by preparative acid-urea-polyacrylamide gel electrophoresis and reversed-phase high performance liquid chromatography. The 4265.0 Da peptide had 38 amino acids, including 6 cysteines. It showed strongest activity against Gram-positive bacteria (Lactococcus lactis subsp. lactis and Staphylococcus aureus; minimum effective concentrations [MECs] 2.4 and 3.0 mu g/ml, respectively) but also had significant activity against Gram-negative bacteria (Escherichia coli D31 and Vibrio parahemolyticus; MECs 7.6 and 15.0 mu g/ml, respectively). Comparison of the amino acid sequence with those of other known antimicrobial peptides revealed that the novel peptide had high sequence homology to arthropod defensins, including those from other bivalves, the mussels A Mytilus edidis and Mytilus galloprovincialis. This is the first antimicrobial peptide to be isolated from any oyster species and we have named it American oyster defensin (AOD). (c) 2005 Elsevier Inc. All rights reserved.