Characterization and genome analysis of a novel isolate of Chryseobacterium sp. associated with spring fish kills in gibel carp (Carassius gibelio)

Recently, an emerging fish disease has caused massive mortality of farmed gibel carp ( Carassius gibelio ) in the early spring, and the diseased fish are associated with red heads and red mouths, which is also called “overwintering syndrome” in China. However, the exact causative agent of this fish kill is unclear. To identify the pathogen of the disease, bacterial isolations were conducted on the diseased gibel carp. The result showed that one dominant strain was identified as Chryseobacterium sp. YC-874, through bacterial morphology, physiological and biochemical tests, and 16S rRNA genetic sequence analysis. Histopathology observation found that the liver, spleen, kidneys, and intestines of the diseased fish had different degrees of damage. Drug sensitivity test showed that the isolate was sensitive to cotrimoxazole, moderately sensitive to enrofloxacin, and resistant to doxycycline, florfenicol, and neomycin. Furthermore, the genome of Chryseobacterium sp. YC-874 was sequenced and analyzed to gain an insight into its pathogenicity. The genome of YC-874 comprised of a single circular 5.32 Mb chromosome with 36.17 % GC content and encoded 4775 coding genes, among which 4643 genes were associated with the pathogenicity, virulence, and effector proteins. Although Chryseobacterium sp. is widely present in aquatic environments, this is the first report of Chryseobacterium sp. isolated from diseased C. gibelio . The infections of fish may be the result of the immunosuppression under spring conditions, combined with the psychrotrophic nature of bacteria. • The isolate associated with Carassius gibelio “overwintering syndrome” in spring. • Fish kills may be the result of immunosuppression under spring conditions. • First isolation of Chryseobacterium sp. from diseased Carassius gibelio . • Chryseobacterium sp. is sensitive to cotrimoxazole. • Genome sequencing and analysis were performed on Chryseobacterium sp.