Pathogenicity and Whole-Genome Analysis of a Siniperca Chuatsi-Derived Nocardia Seriolae Strain

Abstract

Outbreaks of diseases and death of Mandarin fish have been observed in Guangxi, China, since 2023. A bacterial strain (No. 20230510) was isolated from the kidneys of diseased Siniperca chuatsi. Artificial infection tests confirmed that this strain was responsible for the disease observed in S. chuatsi. The bacterium was subjected to morphological observations, pathological analysis, whole-genome sequencing (WGS), and antibiotic susceptibility testing. Artificial infection tests indicated that the mortality rate in high-concentration groups reached 100%, with an LD50 of 3.89×104 CFU/ml, suggesting significant bacterial virulence. WGS revealed that the bacterium consisted of a single circular DNA chromosome, with a total size of 8,123,106 bp and a GC content of 68.14%, comprising 7,638 coding sequences (CDSs), 72 tRNA genes, and 12 rRNA genes. Phylogenetic tree analysis and comparison with the NR database showed that the genome sequence of this strain (accession no. CP130742) had the highest similarity, at 99.98%, with the UTF1 gene sequence of Nocardia seriolae (accession no. GCF_002356035.1), leading to their classification in the same clade. Therefore, this bacterium was identified as N. seriolae. Comparison with the virulence factor database revealed that 403 CDSs in the bacterium’s genome may be virulence genes; these genes were primarily associated with nutrient metabolism, regulatory factors, immune modulation, effector delivery systems, and exotoxins. Comparative analysis of chromosomal gene sequences indicated the presence of multiple types of antibiotic resistance genes. Antibiotic susceptibility testing showed that the bacterium was susceptible to nine antibiotics, including enrofloxacin, doxycycline, florfenicol, and sulfamethoxazole. Histopathological observations revealed varying degrees of chronic granulomatous lesions in the liver, spleen, kidneys, gills, and intestines, with the most severe lesions occurring in the kidneys. These findings confirm that the N. seriolae strain 20230510 is pathogenic and the causative agent of the mortality observed in S. chuatsi, providing crucial data support for further research on comprehensive control strategies against N. seriolae in S. chuatsi.

Keywords: Siniperca chuatsi, Nocardia seriolae, Pathogenicity, Whole-Genome analysis, Antibiotic susceptibility testing