Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment

Biotechnology Developments of Mussel Antibiotic Peptides

Philippe Roch, Guillaume Mitta, André Aumelas, Yin-shan Yang, Alain Chavanieu, Bernard Calas


Cultivated bivalve mollusks—mainly the Pacific cupped oyster Crassostrea gigas, American cupped oyster C. virginica, and native oyster Ostrea edulis, blue mussel Mytilus edulis, and Mediterranean mussel M. galloprovincialis—suffer from various infectious diseases (caused by protozoa, bacteria, and viruses) that threaten their production. Invertebrates do not possess an acquired immunity equivalent to that of vertebrates; however, they do possess a highly efficient defense system. To understand their innate immune reactions would provide useful tools for both disease prevention and genetic selection. In an attempt to explain immunity in bivalve mollusks, we focused on the antimicrobial activity mediated by peptides.

Antimicrobial peptides are part of the innate immunity effector repertoire. Since their discovery in the cecropia moth Hyalophora cecropia by Hans Boman’s group in 1981 (see Boman 1995 for a review), more than 400 peptides have been reported, and others are continuously being discovered. They are most probably present in all living creatures, from plants to mammals. Even if the molecular structures are diverse, some common features allow their classification in a restricted number of families. In addition, peptides with different molecular structures can be similarly involved in the anti-infectious response.

Several small 4-kDa proteins were purified by reverse-phase high-performance liquid chromatography from mussel hemocyte granules (Charlet et al. 1996; Hubert et al. 1996). Based on primary amino acid sequence homologies, mussel peptides were arranged into three families: defensins related to arthropod defensins, mytilins, and myticins, which are originals. In Mediterranean mussel, all the peptides, whatever the family is, possess eight cysteines arranged in specific conserved arrays (Mitta et al. 1999a, 1999b, 2000a). Slight differences in other amino acids determine several isoforms. Genes encoding defensin B and mytilin B have been cloned and sequenced, revealing that both genes share the same organization including four exons and three introns (Mitta et al. 2000b). The solution structure of defensin A was established by 1H-nuclear magnetic resonance and appeared to be closely related to that of flesh fly Phormia terranovae defensin A with an α-helix followed by two antiparallel β-strands linked by three disulfide bonds constituting a typical CS-αβ motif (Yang et al. 2000).

Even sharing protective activities against invading microorganisms, primary sequences, and structures can be very different from one peptide family to another. Tested in vitro against various microorganisms, all peptides inhibited the growth of Gram-positive bacteria. None was active against protozoa or against Gram-negative bacteria, except against some Vibrio species. According to isoforms, defensins, mytilins, and myticins may or may not be active on fungi. All mussel peptides have a bactericidal effect, but with strong differences in kinetics between isoforms. In fact, the different families and isoforms appeared to possess complementary activities, in terms of both target specificity and kinetics. This might explain why mussels present such a diversity of antimicrobial peptides.