Characterization of antibacterial activities and the related mechanisms of a β-defensin in turbot (Scophthalmus maximus)

Abstract

β-defensins (BDs) are small-molecule cysteine-rich cationic antimicrobial peptides that play an important role in innate immunity against microbial invasion. In this study, a β-defensin (SmBD) was identified and characterized from turbot (Scophthalmus maximus). SmBD has an open reading frame containing 192 bp and encodes a protein of 63 amino acids. It has a predicted signal peptide consisting of 20 amino acids and a mature peptide with six conserved cysteine residues forming three disulfide bonds (C1- C5, C2-C4 and C3-C6). The 3D structure prediction shows an N-terminal α-helix and three reverse parallel β-folds. SmBD showed high sequence identity (70.45% - 95.24%) with β-defensin 2 group members from other teleost species. SmBD had the highest expression in skin of healthy turbot and was significantly up-regulated in mucosal tissues (skin, gill, intestine) after bacteria challenge. SmBD showed antibacterial activity against Vibrio anguillarum, Edwardsiella tarda, Escherichia coli and Streptococcus iniae, and inhibitory effect on the formation of bacterial biofilms. SmBD changed bacterial membrane permeability and affected bacterial morphology, as shown in fluorescence microscopy and scanning electron microscopy analyses, respectively, which indicated that SmBD might exercise antibacterial behaviors via “membrane-attack” mechanism.