Terminal carbohydrate composition, IgM level and enzymatic and bacteriostatic activity of European sea bass (Dicentrarchus labrax) skin epidermis extracts

• First study comparing some immune activities present on fish skin epidermal extracts.
• Aqueous and organic skin extracts had important molecules involved in mucosal immunity.
• IgM levels in aqueous and organic extract are very similar.
• Aqueous extract could be more suitable for measuring some activities.
• Organic extract has a notable bactericidal activity of all the bacterial strains tested.

Although the skin is one of the main defense barriers of fish to date, very little is known about the immune implications and the properties of the numerous substances present in skin cells. In the present study, terminal carbohydrate composition and some components of the skin immunity (total IgM level, and several enzymatic and bacteriostatic activities) present on aqueous and organic epidermal extracts of European sea bass (Dicentrarchus labrax) were determined. Most of the parameters measured followed a protein concentration dose-response. Curiously, both skin extracts have similar levels of total IgM. However, aqueous extracts showed higher presence of some terminal carbohydrates, alkaline phosphatase and esterase activities and lower proteases and ceruloplasmin activities than epidermal organic extracts. Regarding the bacteriostatic activity, the growth of all the bacterial strains tested was reduced when cultivated in presence of organic extracts, being the observed reduction correlated to the protein concentration present in the extract sample. On the contrary, skin aqueous extracts have no significant effect on bacterial growth or even allow bacteria to overgrow, suggesting that the bacteria could use the extracts as a nutrient source. The results are discussed and compared with the same activities studied on fish skin mucus in order to understand their possible implications on mucosal immunity.