With a production of around 250,000 tons per year, France is Europe’s leading shellfish producer. Although conchyliculture is often described as a sustainable alternative to wild fisheries in the context of growing anthropic pressures, it generates a tremendous amount of shells, mostly considered as waste. While some efforts have been made to recycle it, this very abundant and cheap co-product has yet a low added-value: it is merely used for construction, soil amendment or as poultry feed calcium supplements. As the first shield against chemo-physical and biological threats, mollusc shells are formed through a highly controlled process involving the secretion - by the mantle - of macromolecules that regulate the deposition of the crystalline units and remain occluded in the mineral phase. The development of high-throughput techniques like transcriptomics and proteomics reveal the complexity of this calcifying matrix and further supports the hypothesis that the molluscan shell is not solely a physical barrier. Indeed, the presence of highly basic peptides suggests potential bactericidal properties. Antimicrobial peptides are some of the most effective innate immune system molecules in metazoans. Unlike traditional antibiotics, they do not induce any form of resistance: among the molecular processes often involved, destabilisation of bacterial membranes and their permeabilization constitute an unstoppable mechanism for bacteria. To date, bactericidal agents have only been identified in soft tissue in molluscs, but their putative presence in the shells may partly explain certain natural observations: although bivalve shells remain in aqueous environments conducive to microbial contamination, they are not very prone to the development of bacterial biofilms.
Here, we seek to demonstrate in vitro the antimicrobial capacity of calcifying shell matrices extracted from commonly consumed molluscs in Europe through isolation and characterization of these macromolecules. For this purpose, organic matrices were extracted from whole-shell powders and fractionated according to their solubility in dilute cold acetic acid, and, for the soluble fraction only, according to different ultrafiltration cutoffs. Common strains of marine pathogenic bacteria were selected (Aliivibrio salmonicida, Vibrio tapetis, V. mytili, V. harveyi, V. aestuarianus) and the effects of different shell extracts on bacterial growth were measured via disk diffusion assays. In parallel, all organic fractions were analysed by proteomics (LC-MS-MS). Positive results were obtained, showing bacterial growth inhibition of A. salmonicida and V. harveyi in the presence of insoluble matrices, in particular that of the oyster and of the cockle. Considering the role - sometime cooperative - of these strains in vibriosis outbreaks, and the rise of antibiotic resistance among Vibrio species, our results could lead to innovative and non-polluting applications in aquaculture, food safety, conservation science and health. This project intends to recycle co-products and make the most of them, therefore helping to establish a virtuous circular economy and a more sustainable aquaculture.