Adhesion of different virulent strains of Flavobacterium psychrophilum to the gut tissue of the rainbow trout (Oncorhynchus mykiss)

Objective: Flavobacterium psychrophilum is the causative agent of bacterial cold water disease (BCWD) and rainbow trout fry syndrome (RTFS) in salmonids. So far, there is limited information available about the portal of entry of this bacterium. Method & Materials: In this study, the ability of a low D2 (99/10A) and two high [Dubois and D1 (950106-1/1)] virulent strains of F. psychrophilum to adhere to gut tissue of rainbow trout were evaluated. A gut perfusion model was adopted, offering a number of advantages compared to other in vitro as well as in vivo models. The perfused gut segments of fish were injected with low and high virulent strains of F. psychrophilum and were incubated in an aerated bath for 60 and 90 min. After incubation, samples of the gut were taken for bacteriological, histological and electron microscopical examinations. Results & Conclusion: Bacteriological examination proved that the number of gut-associated F. psychrophilum was higher for Dubios and D1 than for D2. Histological samples clearly showed numerous bacteria of the high virulence strains associated with the intestinal tissue as opposed to only few bacteria of the low virulence strain. Extensive exfoliation of intestinal epithelium was also noted after incubation with the high virulence strains, but less with the low virulence strain. These findings were also confirmed using scanning electron microscopy (SEM). Additionally, the higher capacity of F. psychrophilum to adhere to gut tissue at lower temperatures as observed in the present studies. In conclusion, these results suggest that the intestinal epithelium might represent an important site for adherence and colonization of F. psychrophilum strain and may act as a portal of entry. Moreover, the temperature had a negative influence on the adherence process.

Keywords:

Flavobacterium psychrophilum , adhesion , perfused gut , portal of entry , Oncorhynchus mykiss