Characterization of the adhesion of Flavobacterium columnare (Flexibacter columnaris) to gill tissue

Flavobacterium columnare (Flexibacter columnaris) is an important cause of gill and skin disease in freshwater fish species, often causing high mortal ity. In previous studies, virulence of F. columnare was correlated with the ability to adhere to the gill tissue. To gain insight into the factors res ponsible for adherence, a gill perfusion model was used. The bacterial cell s of the high virulence strain AJS 1 were exposed to various treatments, af ter which they were added to the organ bath of an isolated gill arch and ad herence to the gill tissue assessed. Adherence capabilities were significan tly reduced following treatment of the bacteria with sodium metaperiodate o r incubating them with d-glucose, N-acetyl-d-glucosamine, d-galactose and d -sucrose. Incubation of the bacteria with trypsin and pronase did not signi ficantly inhibit adherence. The binding sites for F. columnare on the gill tissue were also partially characterised. Treatment of the gill with sodium metaperiodate reduced adhesion, but treatment with pronase or trypsin did not cause any significant reduction, indicating that the major component of the receptor is of carbohydrate nature. Adherence ability of the bacteria correlated well with their haemagglutination capacity using chicken and gui nea pig erythrocytes. Higher haemagglutination titres were obtained with th e highly virulent strain AJS 1 than with strain AJS 4, a strain with low vi rulence and adherence capacity. Haemagglutination was partially inhibited a fter incubation of the bacteria with d-glucose and N-acetyl-d-glucosamine a nd after treatment of the bacteria at 41 degrees C for 10 min (minor heat t reatment). It was completely abolished following incubation of the bacteria l cells with sodium metaperiodate and intensive heat treatment (65 degrees C, 25 min). Haemagglutination was also in-sensitive to pronase and trypsin treatment. Transmission electron microscopy (TEM) revealed that the high vi rulence strain had a thick capsule (120-130 nm) with a regular, dense appea rance, whereas the capsule of the low virulence strain was much thinner (80 -90 nm) and less dense. TEM also demonstrated the loss of the capsule of th e high virulence strain after treatment of the bacterial cells with minor h eat and sodium metaperiodate. These results indicate that a lectin-like car bohydrate-binding substance incorporated in the capsule is responsible for the attachment of F. columnare to the gill tissue.