The association of Flavobacterium columnare strains of high and low virulence with gill tissue of black mollies (Poecilia sphenops)

The ability of a high virulence strain (AJS 1) and a low virulence strain ( AJS 4) of Flavobacterium columnare (Flexibacter columnaris) to attach to th e gills of black mollies (Poecilia sphenops) was investigated, For that pur pose, two groups of 15 black mollies each were immersed in a bath containin g 10(6) CFU/ml of F. columnare AJS 1 or AJS 4. At regular intervals from 1 to 12 h after the contact infection, fish were sacrificed and gills, skin, spleen and heart were sampled for bacteriology. Samples of the gills were t aken for immunohistochemical and electron microscopic examination. Bacterio logical examination proved that the number of gill-associated F: columnare was higher for AJS 1 than for AJS 4. Strain AJS 1 was isolated from the hea rt and spleen of 6 and 1 of the 16 examined animals, respectively. Strain A JS 4 was not isolated from the internal organs of any fish. When examined i mmunohistochemically. strain AJS 1 was found closely associated with gill e pithelium whereas this was not the case for strain AJS 4. The adherence of bacteria to the gill tissue challenged with the virulent strain AJS 1 was a lso clearly demonstrated using scanning and transmission electron microscop y. These results indicate that adhesion of F. columnare to the gill tissue constitutes an important step in pathogenesis. (C) 1999 Elsevier Science B. V. All rights reserved.