Isolation, Identification and Whole-Genome Sequencing of a Nocardia seriolae Strain from Farmed Chinese Rice-Field Eels (Monopterus albus).

Simple Summary: The Chinese rice-field eel (Monopterus albus) is an important freshwater fish widely cultured in China. In recent years, disease outbreaks have caused heavy economic losses to its aquaculture. In October 2025, an outbreak of severe disease with high mortality occurred among farmed individuals in Jiangxi Province. Infected fish showed obvious white nodules in visceral organs and severe tissue lesions. This study first isolated Nocardia seriolae from diseased individuals and confirmed it as the causative pathogen. Artificial infection experiments verified its high pathogenicity to the Chinese rice-field eel. We also analyzed its genomic characteristics and antibiotic susceptibility profile. These results provide useful descriptive information and reference for the diagnosis and identification of this disease in the Chinese rice-field eels. In October 2025, an outbreak occurred among farmed Chinese rice-field eels (Monopterus albus) in Jiangxi, China. A Nocardia seriolae strain designated JXMa251025, which has not been previously documented in Chinese rice-field eels, was isolated from moribund fish exhibiting multiple white nodules of various sizes in visceral tissues. Histopathological examination revealed multi-organ damage, including necrosis of liver cells, granulomatous inflammation with hemorrhage in visceral organs, and necrosis of renal glomeruli and tubules accompanied by vascular congestion. Artificial infection trials confirmed that strain JXMa251025 reproduced clinical signs consistent with those observed in the natural outbreak. Infection experiments resulted in 100% mortality in high-concentration challenge groups, with a median lethal dose (LD50) of 9.76 × 10⁵ CFU/mL, indicating high virulence. Whole-genome sequencing revealed a circular chromosome of 8,295,032 bp with a GC content of 68.10%. The genome contains 66 tRNA genes and four copies each of the 23S, 16S, and 5S rRNA genes. Phylogenomic analysis placed strain JXMa251025 within a clade of Nocardia seriolae strains with approximately 99% bootstrap support, confirming its identification as Nocardia seriolae. Further genomic screening identified 253 potential virulence genes associated with nutrient metabolism, regulatory systems, immune modulation, effector delivery, and exotoxin production. Antibiotic susceptibility testing showed that strain JXMa251025 was sensitive to seven antibiotics: ciprofloxacin, neomycin, enrofloxacin, florfenicol, gentamicin, amikacin, and doxycycline. This study represents the first report of Nocardia seriolae infecting Chinesse rice-field eels, providing useful descriptive information for disease diagnosis and reference. [ABSTRACT FROM AUTHOR]

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