Emerging pathogen Chryseobacterium sp. isolated from diseased goldfish Carassius auratus: antimicrobial...

Chryseobacterium sp., belonging to the family Weeksellaceae, is an emerging fish pathogen that is opportunistic in nature. In this study, we investigated a clinically infected goldfish Carassius auratus from a fish farm during a disease outbreak. The bacterial colonies were isolated from the kidneys of infected goldfish. Following a 24 h incubation at 28°C, the colonies on nutrient agar exhibited yellow pigmentation and were identified as Gram-negative, rod-shaped, catalase-positive, and oxidase-positive. A total of 35 antibiotics belonging to different classes were used to determine the antibiotic susceptibility pattern of the isolated bacteria. Amplification and sequence analysis of the 16S rRNA gene identified the microorganism as Chryseobacterium sp., sharing 99.3% sequence similarity. The pathogenicity of this bacterial pathogen was experimentally confirmed in goldfish by a challenge experiment, and LD50 was determined as 7.46 × 107 CFU ml-1. Histopathological analysis of the infected fish revealed degeneration of renal tubule epithelium and melano-macrophagic aggregates with hyperaemic condition in the kidney. Similarly, infected gills exhibit primary and secondary lamellar hyperplasia, with lamellar oedema, necrosis and fusion of secondary lamella. This study underscores the virulence of Chryseobacterium sp. as a significant opportunistic pathogen in goldfish, highlighting the need for vigilant monitoring and intervention strategies to prevent outbreaks.