Characterization and pathogenicity study of Chryseobacterium scophthalmum recovered from gill lesions of diseased golden mahseer, Tor putitora (Hamilton, 1822) in India

Eighteen orange pigment bacterial strains have been recovered from the gill of naturally diseased golden mahseer (n = 9), Tor putitora (Hamilton, 1822) in Uttarakhand, India. Isolated strains were characterized by morphological and biochemical characteristics and 16S rRNA gene sequence, and were found to belong to the homogenous group. All the bacterial isolates were Gram-negative, flexirubin positive, catalase and cytochrome-oxidase positive, gliding, short (~ 1.9-2.7 μm in length) and straight rods. The 16S rRNA gene sequence analysis allocated the bacterium to taxa Chryseobacterium scophthalmum (GenBank accession nos. KM822770, type strain C. scophthalmum TPBLGL 18), with C. scophthalmum strain LMG 13028T as its closest relative, with 99% sequence similarity. Antibiotic susceptibility tests indicated that all the isolates were susceptible to oxytetracycline (2 μg), erythromycin (10 μg) and sulphadiazine-trimethoprim (100 μg). Histopathological study of the gill of naturally diseased golden mahseer revealed extensive epithelial cell hyperplasia, necrosis, fusion of secondary gill lamellae, degeneration of chloride cells and edema. Inflammation and degenerative changes were also observed in liver, heart, spleen, gall bladder and kidney of naturally diseased fish. The pathogenicity of C. scophthalmum TPBLGL 18 (representative strain) to golden mahseer was confirmed by intraperitoneal (i.p.) injection (0.1 mL volume) with 1.9 × 106 CFU/mL of bacterium, which resulted in appearance of gill lesions. Gill lesions in 75% of the challenged fish were reproduced 24 h post-injection (p.i.), and pure colony of C. scophthalmum were re-isolated from the gill, liver and kidney tissue of experimentally challenged golden mahseer. Histopathological studies of gill tissues of i.p injected golden mahseer revealed, clubbing, hyperplasia, hypertrophy, congestion and fusion of secondary gill lamellae. Therefore, in conclusion, our study demonstrates the virulence of C. scophthalmum in golden mahseer as the causative agent of bacterial gill disease (BGD).