Isolation, identification, and histopathological observation of pathogen causing nocardiosis in Channa argus

To explore the pathogenic mechanism of nocardiosis in Channa argus, we studied the pathogenicity and drug sensitivity of the pathogen and the immune resistance of C. argus by isolation and identification of the pathogen, histopathology and gene expression analysis. The results showed that the diseased C. argus was mainly infected with a pathogen strain SDAT 0011. The strain SDAT 0011 was identified as Nocardia seriolae using the morphologic structure and staining observation, PCR amplification of species-specific primers, sequence analysis of 16S rRNA gene, and physiological and biochemical tests. The challenge experiments showed that the isolated strains were pathogenic on C. argus and the mortality rate of the 1 × 10⁵ − 1 × 10⁸ CFU/mL injection groups was 100%. In addition, the diseased C. argus exhibited numerous marked white nodules (round/ovoid), ranging from 0.1 to 0.2 cm in diameter on internal organs, especially the spleen, kidney, and liver. Histopathological observation and analysis showed that the structure of chronic granuloma was visible, with many lymphocytes, damaged or dead cells in the center. The antibiotic susceptibility assays of the strain SDAT 0011 showed that the strain was sensitive to rifampin, but resistant to penicillin, cefradine, and ampicillin, etc. Furthermore, the expression levels of toll-like receptors 2 gene (TLR2), toll-like receptors 13 gene (TLR13), and C-C chemokine receptor type 9 gene (CCR9) after infection were analyzed by real-time fluorescence quantitative PCR (qRT-PCR). In our study, following N. seriolae challenge, TLR2 and TLR13 were significantly up-regulated, while CCR9 was significantly down-regulated at all examined time points in the spleen and head kidney. These results implied that some genes of the toll-like receptor signaling pathway and the chemokine signaling pathway have been activated in the early stages of infection to resist Nocardia infection. The results of this study will lay a foundation for the treatment of N, seriolae and studying it pathogenesis..