Isolation and Identification of Nocardia seriolae from Large-mouth Bass (Micropterus salmoides) and Pathological Lesions of Its Infection

Purpose In recent years, a serious infectious disease characterized by skin ulcers and white nodules in visceral organs emerged in Micropterus salmoides farms in Sichuan. Therefore, we analyzed its etiology and pathological characteristics.

Method The isolation of pathogenic bacteria, artificial infection, physical and chemical characteristics of the isolated bacteria, 16S rDNA and hsp65 gene sequence analysis, drug sensitivity test and pathological damage observation were carried out in this study.

Result Three strains of G⁺ filamentous bacteria with weak acid resistance were isolated from the viscera of diseased fish. Artificial infection test confirmed the pathogenicity of the isolated bacteria, and their physical and chemical characteristics were identical with the type strain (ATCC43993). In the phylogenetic trees based on 16S rDNA and hsp65 sequence, 3 isolates and N. seriolae ATCC43993 constituted a branch, and 3 isolates were identified to be N. seriolae. 3 isolates were sensitive to doxycycline, erythromycin and gentamicin, and resistant to penicillin and ceftiofur, and there were some differences in enrofloxacin, ofloxacin and trimethoprim/sulfemethoxazole. Histopathological injuries of large mouth perch infected with N. serioloe mainly manifested as chronic granulomatous nodules of varying sizes in multiple tissues and organs of the whole body. Typical nodules were generally divided into three layers: caseous necrosis, epithelioid cells, lymphocytes and fibroblasts peripherally. And gill, liver, spleen, heart, kidney and head kidney were severely damaged. A large number of pathogenic bacteria were distributed in granuloma.

Conclusion N. seriolae infection is the cause of the largemouth bass, which causes the formation of granulomatous nodules in many tissues and organs, and there are a large number of pathogenic bacteria in the nodules.