Phenotypic and molecular characterization and pathology of Flectobacillus roseus causing flectobacillosis in captive held carp Labeo rohita (Ham.) fingerlings

•A new bacterial disease flectobacillosis caused by Flectobacillus roseus in Labeo rohita is reported.•F. roseus RF1 produced pink to rose pigmented colonies on selective cytophaga agar.•It formed a monophyletic branch with Flectobacillus sp. KC588923 and F. roseus EU921645.•The inflammation type following induced flectobacillosis in L. rohita was granuloma-like.•F. roseus RF1 produced systemic pathology in L. rohita kidney when challenged.

A new bacterial disease (flectobacillosis) caused by Gram negative, long rod shaped Flectobacillus roseus in captive held carp, rohu (Labeo rohita) in West Bengal, India is described. Besides the phenotypic and molecular characterization, histopathology of F. roseus of naturally infected and challenged rohu were studied. The affected rohu had loss of mucus and scale, discoloured body, peeled skin and rotten tail, but no signs of gill damage. On selective cytophaga agar, F. roseus produced pink to rose pigmented colonies. Phylogenetically, F. roseus RF1 formed a monophyletic branch with F. roseusEU921645 (96% sequence similarity) and Flectobacillus sp. KC588923 (94% sequence similarity). It caused significant mortalities in rohu when challenged. The naturally infected rohu showed epithelial disruption, inflammation and necrosis of skeletal muscle tissue accompanied by haemocyte infiltration. The kidney of challenged rohu showed proximal and distal tubule degeneration, necrosis, thickening of epithelial layer of proximal and distal tubules, necrotised haematopoietic tissue, nephritic tubule with widened lumen, vacuolation of tubular epithelium, glomerular congestion, desquamation, glomerulopathy with dilated Bowman's space, cellular and nuclear hyperprophy, pycnotic nuclei, melano-macrophage aggregates and granuloma-like inflammatory response. These results suggest that F. roseus can produce systemic pathology in kidney similar to many other known fish pathogens.