Comparative assessment of organic solvent extraction on non-specific immune defences of skin mucus from freshwater fish

Fish skin mucus secretion is an important strategy against pathogens since it contains several immune molecules that act as the first line of defence. To date, several studies have reported that the mucus composition and immune responses vary depending on the fish species, and consequently, the comparative studies on skin mucus may have beneficial applications in the field of aquaculture. Therefore, the aim of the present study was to characterize functional groups of skin mucus collected from three different freshwater fish: common carp (Cyprinus carpio), rohu (Labeo rohita), and mrigal (Cirrhinus mrigala) and compare the antibacterial activity and innate immune parameters after organic solvent (acetone and methanol) extraction. Firstly, the Fourier transform infrared (FT-IR) spectral analysis of crude skin mucus demonstrated that the three fish species showed similar functional groups. Both the organic solvent extracts from skin mucus of three fish species exhibited antibacterial activity. Interestingly, skin mucus methanol extract from mrigal showed higher antibacterial activity when it was incubated with pathogenic bacteria tested and compared to the results found in the other fish skin mucus extracts. Regarding the innate immune-related enzymes, the lysozyme exhibited higher activity in the methanol extract of mrigal fish skin mucus compared to acetone extract as well as the other extracts of skin mucus from common carp and rohu fish. Alkaline phosphatase activity was significantly higher in skin mucus methanol extracts of common carp and mrigal fish compared to the acetone extract of the same species. In the case of protease enzyme, the activity observed was significantly higher in the skin mucus methanol extract compared to acetone extracts of all the three fish species, being the highest protease activity in the methanol extract of mrigal skin mucus. Therefore, our results demonstrated that the methanol extract of skin mucus displayed higher antibacterial and innate immune-related enzymes activities compared to acetone extract. When we compared to the species, the mrigal skin mucus extracts exhibited greater activities than common carp and rohu fish. These findings suggest that the methanol extract could be useful to isolate more bioactive molecules than the acetone extract in the species studied, which could be useful for therapeutic applications in aquaculture.