Characterization Of Flavobacterium Columnare From Farmed Infected Rainbow Trout, Oncorhynchus Mykiss (Walbaum, 1792) Of Central Indian Himalayan Region, India

Flavobacterium columnare, the causative agent in columnaris disease, leads to the mass mortality in many cultured freshwater fish species globally. Flavobacterium columnare (n = 7) was isolated from the gill lesions of farmed rainbow trout, Oncorhynchus mykiss in Central Indian Himalayan region. The bacterium was identified by colony characteristics, partial amplification of 16S rRNA gene and phylogenetic analysis. The partially amplified 16S rRNA gene (similar to 1400 bp) was sequenced and matched with (>99%) the sequences of other F. columnare strains in the GeneBank and designated as F. columnare, BRTAGILLFC 01. From phylogenetic analysis of 16S rRNA sequences of F. columnare, the isolate was assigned to genetic group 2. The in-vitro assays for the virulence characteristics of the F. columnare BRTAGILLFC 01 revealed its ability for hemolysis, biofilm formation, protease production and capsule formation. In transmission electron microscopy (TEM), the negatively stained bacterial samples revealed the presence of capsular structure and large numbers of fimbriae. The experimental infection of rainbow trout fingerling with F. columnare BRTAGILLFC 01 at 1.5 x 10(8) CFU mL(-1) reproduced the disease and led to cumulative mortality of 60% within 30 days. The post challenge observations illustrated swelling and hemorrhage in gill, abnormal behavior and pigmentation, decreased feeding and increased mortality in infected rainbow trout fingerlings. The histopathology of trout depicted severe degenerative changes in gill such as necrosis, fusion of secondary gill lamellae, hypertrophy, hyperplasia and epithelial lifting. The other organs revealed cellular hypertrophy, congestion and vacuolation in liver, renal corpuscle dilation and tubular necrosis in kidney and melanomachrophage aggregation in spleen. Among all therapeutic antibiotics, the bacterium was susceptible to oxytetracycline, erythromycin, ampicillin and florfenicol at the lower concentrations. In conclusion, the study confirms the first report of F. columnare infection in gill tissue of farmed rainbow trout in Central Himalayan region of India.