RESPONSE OF PASTEURELLA-PISCICIDA AND FLEXIBACTER-MARITIMUS TO SKIN MUCUS OF MARINE FISH

The antibacterial activity present in the skin mucus of turbot Scophthalmus maximus, seabream Sparus aurata and seabass Dicentrarchus labrax against Pasteurella piscicida and Flexibacter maritimus was evaluated. Using assays on agar plates, none of the mucus samples from the above fish showed any antibacterial activity against F. maritimus isolates. Turbot mucus inhibited the growth of the P. piscicida but mucus from seabream and seabass did not. Assays in liquid systems to determine the survival of the above pathogens in the presence of skin mucus corroborated the results obtained by the agar plate method. The bactericidal properties of the mucus were lost after heat treatment at pH 3.5 and all skin mucus samples displayed activity against Staphylococcus aureus ATCC 25923, a strain resistant to lysozyme. These findings indicated that thermolabile substances other than lysozyme were responsible for the antibacterial activity in mucus of marine fish. Enzymatic and heat treatments of the mucus also showed that factors other than complement were involved and that the active component(s) was likely a glycoprotein. Regardless of the source of isolation and degree of virulence, all P. piscicida and F. maritimus strains adhered strongly to the skin mucus of the 3 fish species tested. Taking all of the foregoing results into consideration, it appears that whereas a possible portal of entry for F. maritimus into the fish body is the skin, in P. piscicida another pathway must be involved.