Pathogenicity and whole genome analysis of a Nocardia seriolae strain isolated from snakehead

Abstract:

[Background] The infection of Nocardia seriolae, a typical opportunistic pathogen, has been documented in many kinds of fishes such as Channa argus and Micropterus salmoides, particularly wounded fishes or fishes with decreased immunity. The resulted nocardiosis lasts a long time, causing great loss to the aquaculture. [Objective] This paper aims to clarify the pathogenicity of Nocardia seriolae isolated from hybrid snakehead (C. maculata♀×C. argus♂), as well as the whole genome and virulence factors information of the pathogen, which is expected to lay a basis for the future research on etiology ofNocardia seriolae, prevention and control of this pathogen, and vaccine development. [Methods] Through challenge test and pathological analysis of target snakehead organs, the virulence and pathogenic characteristics of Nocardia seriolae NK201610020 were elucidated. In addition, based on whole-genome sequencing and comparative genomics analysis, the genomic features and virulence genes information of the strain were clarified. [Results] The result of challenge test showed that the death rates in the five infection groups were up to over 90% except the group of 1.5×10³, and the LD₅₀is1.079×10³ CFU/mL, which indicating the strong virulence of the strain. Histopathological results revealed serious pathological damages in liver, spleen, and kidney, as well as formation of granulomas. The whole genome of NK201610020 was 8 294 329 bp with GC content of 68.10% and 7 812 coding genes. The genomes of Nocardia seriolae from different hosts and different regions showed little difference (>99.9% similarity). According to the alignment with virulence gene database, 171 coding genes in the genome of NK201610020 might be virulence genes, which were involved in cell wall synthesis, nutrient metabolism, and persistent bacterial infection. [Conclusion] Nocardia seriolae has strong virulence, and conserved genome sequence containing diverse virulence genes. The results lay a basis for further analysis of Nocardia seriolae pathogenic mechanism.