Characterization and pathological analysis of Flavobacterium tructae recovered from farmed rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), in the Indian Himalayan Region

Flavobacterium species have emerged as a significant pathogen in aquaculture worldwide. Therefore, there is a need to investigate the occurrences of Flavobacterium species and their pathogenicity in aquaculture. In this study, gram-negative, short rod-shaped isolates of Flavobacterium tructae , positive for the flexirubin pigment test (30% KOH) and exhibiting gliding movement, were recovered from the caudal peduncle of farmed infected rainbow trout, Oncorhynchus mykiss , in the Indian Himalayan Region. The isolates were characterized by biochemical, 16S rRNA, phylogenetic analysis, and designated as laboratory strain RTFCPB 279 and submitted to NCBI (MT896143.1). Experimental infection of rainbow trout fingerlings with F. tructae RTFCPB 279 at 10 8 CFU mL −1 demonstrated clinical signs of disease progression, leading to nearly 50% cumulative mortality. The survival of the bacterium in blood serum, hemolytic activity, and biofilm formation were indirectly correlated with its virulence characteristics. The serum-enzyme assay showed a significant increase in aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities in the liver of infected trout. The histopathological assessment of the gill, liver, kidney, spleen, and muscle tissues of rainbow trout collected from both natural and experimental infections showed several degenerative changes. The test bacterium was found to be sensitive to macrolides, phenicols, tetracycline, aminoglycosides, and folate pathway inhibitors, indicating the potential therapeutic use of these antibiotics. The lowest MICs of 4, 16, and 32 μg mL −1 were recorded against the FDA-approved erythromycin, oxytetracycline, and florfenicol, respectively. The study concludes that F. tructae RTFCPB 279 is an opportunistic pathogen and the correlation between environmental factors and infection dynamics of F. tructae may require further investigation to evaluate its actual risk in global aquaculture. The prudent use of erythromycin and oxytetracycline can help control the infection of F. tructae in rainbow trout culture. Highlights • Flavobacterium tructae , an opportunistic pathogen in the flavobacteria group, was isolated from farmed rainbow trout in the Indian Himalayan Region. • In the experimental infections of rainbow trout fingerlings at a concentration of 10 8 CFU mL -1 , the bacterium demonstrated moderate pathogenicity. Additionally, the bacterium exhibited in vitro biofilm formation and haemolytic activity. • Antimicrobial susceptibility assays demonstrated F. tructae's sensitivity to various antibiotic classes, including macrolides, phenicols, tetracycline, aminoglycosides, and folate pathway inhibitors. • Among the approved antibiotics for aquaculture, erythromycin, florfenicol, and oxytetracycline recorded the lowest minimum inhibitory concentrations (MICs) against the bacterium