First isolation of Chryseobacterium sp. as a pathogen of fish eggs

In recent years, disease has become a major limiting factor in the development of the tilapia fry culture industry. This study isolated eight dominant bacterial strains, designated A8PD1 to A8PD8, from diseased Nile tilapia ( Oreochromis niloticus ) eggs. Among them, strain A8PD8 exhibited the highest pathogenicity, causing up to 70 % egg mortality and deformities in hatchlings. Infection experiments demonstrated that increasing concentrations of A8PD8 led to higher egg mortality and deformity rates, with an LD 50 of 2.03 × 10⁶ CFU/mL. Pathological analysis confirmed that A8PD8 infection damaged egg membrane integrity and disrupted embryonic development. Morphological, physiological, biochemical, 16S rRNA and genome-wide analyses identified A8PD8 as a Gram-negative bacillus with closest similarity to Chryseobacterium. urinae . Antibiotic susceptibility testing showed that the bacterium was most sensitive to clindamycin (MIC=1 μg/mL) and most resistant to ceftazidime (MIC>256 μg/mL). Whole-genome sequencing revealed that the A8PD8 genome is 4,480,960 bp, with a GC content of 35.72 %, encoding 285 virulence genes and 169 resistance genes. This is the first report of a pathogenic Chryseobacterium strain affecting fish eggs, providing a scientific foundation for preventing and controlling Chryseobacterium infections during tilapia egg incubation. • The pathogenic bacterium A8PD8 of the genus Chryseobacterium was isolated from fish eggs for the first time. • A8PD8 is a pathogen of tilapia eggs, but not a pathogen of tilapia larvae. • Infection of tilapia eggs with A8PD8 led to egg mortality or the hatching of deformed larvae. • Whole - genome sequencing revealed that the A8PD8 genome encoded 285 virulence genes and 169 resistance genes.