Abstract
Glycophorins (GPs) in red blood cell (RBC) membranes of carp (Cyprinus carpio L.) exhibit bacteriostatic activity against various gram-negative and gram-positive bacteria including fish pathogens. This physiological property also exists in the GPs of yellow tail (Seriola quinqueradiata) and red sea bream (Pagrus major). Thus, we concluded that this antimicrobial activity is not confined to these teleost species but can be found in all fish. This bacteriostatic activity is caused by the sialo-oligosaccharide from these teleost GPs. Only the N-glycolylneuraminic acid (NeuGc) form of sialic acid was detected in the carp. Using NMR and GC–MS, we determined that the structure of the bacteriostatic sialo-oligosaccharide from carp was NeuGcα2→6 (Fucα1→4) (Glcα1→3) Galβ1→4GalNAc-ol. The bacteriostatic activity of this monosialyl-oligosaccharide is due to the property of the lectin receptor. It is supposed that some lectin-like proteins exist on the surface of gram-positive bacteria or the flagellum of gram-negative bacteria. Based on the electron microscope observations, teleost GPs containing the sialo-oligosaccharide are released from RBC membranes and then adsorbed onto the surface or the flagellum of invading bacteria in the blood.