In situ adherence of Vibrio spp. to cryosections of Atlantic salmon, Salmo salar L., tissue.

Pathogenic and presumed non‐pathogenic bacteria isolated from fish were tested for their adhesion to cryosections from different mucosal surfaces of Atlantic salmon, Salmo salar L. Adhered bacteria were detected by immunohistochemistry. Mucus was stained and fixed with Alcian blue after incubation of bacteria. The majority of the bacteria tested, i.e. Vibrio anguillarum serotype O1, Vibrio salmonicida, Vibrio viscosus, Flexibacter maritimus and ‘gut vibrios’, i.e. Vibrio iliopiscarius and intestinal isolates of V. salmonicida, all adhered to mucus on all salmon epithelial surfaces tested, including sections from the foregut, hindgut, pyloric caeca, gills and skin. In contrast, V. anguillarum serotype O2, including both serotypes O2a and O2b, did not adhere to mucus, but did adhere to all other components of the tissues. As a positive control for adhesion of bacteria on cryosections, Escherichia coli was bound to piglet ileal mucosal lining, and as a negative control for adhesion, Staphylococcus aureus was found not to bind to any of the tissues tested. The present study shows that adhesion to mucus was not restricted to pathogenic bacteria, and furthermore, that not all pathogenic bacteria studied adhered to mucus. Hence, on the basis of these findings, the present authors suggest that V. anguillarum O2 may have an invasion strategy which does not involve adhesion to mucus, and thus, differs from the other pathogenic bacteria in the present study, which all bound to salmon mucus.