Pathological analysis and antimicrobial susceptibility of Chryseobacterium balustinum RTFCP 298 isolated from diseased rainbow trout, Oncorhynchus mykiss

Abstract

In this study, six isolates of Chryseobacterium balustinum were characterized from diseased rainbow trout fngerlings. The virulence characteristics, pathogenicity, and antimicrobial susceptibility pattern of these isolates were investigated. The bacterium showed positive results for catalase, cytochrome oxidase, and aesculin hydrolysis, while negative results were obtained for DNase, gelatinase, methyl red, Voges-Proskauer’s reaction, Simon citrate, Hydrogen sulphide, and starch hydrolysis. Amino acid metabolism analysis revealed the inability to metabolize arginine, lysine, and ornithine decarboxylase. Molecular characterization (16S rRNA) and phylogenetic analysis revealed the test isolates as C. balustinum, closely related to strain WLT (99.85% similarity) and C. balustinum P-27 (99.77%). Virulence assay indicated haemolytic activity and bioflm formation by the test bacterium. The challenge test confrmed moderate pathogenicity in rainbow trout and established Koch’s postulates. The clinical manifestations of infection included fn erosion, eye and body surface haemorrhage, exophthalmia, and organ liquefaction. The minimum inhibitory concentrations of various antimicrobials ranged from 1 to> 256 µg ­mL−1. The novel synthetic antimicrobial peptides exhibited MICs of 8 to> 256 µg ­mL−1, suggesting a potential control method. These fndings suggest that C. balustinum is an opportunistic pathogen with moderate pathogenicity in rainbow trout. Further research on the host–pathogen relationship is necessary to understand virulence characteristics and pathogenicity in aquaculture.