Co-Infection of Aeromonas veronii and Citrobacter braakii in Yellow Catfish (Tachysurus fulvidraco).

Recently, a disease outbreak was observed in farmed yellow catfish (Tachysurus fulvidraco) in Tianjin, China. Naturally infected fish exhibited surface haemorrhages, abdominal distension, anal redness, and hemorrhagic ascites. Histopathological examination revealed systemic lesions in the stomach, intestine, liver, and spleen. Two Gram-negative bacterial isolates, designated PFL and PFS, were isolated from the liver and spleen of diseased fish, respectively. The isolates were identified as Aeromonas veronii and Citrobacter braakii by morphological and biochemical characterisation combined with 16S rRNA and gyrB gene sequence analyses. Experimental infection with single or combined isolates demonstrated that both A. veronii and C. braakii were pathogenic to yellow catfish, with A. veronii showing higher virulence. Fish infected with A. veronii or co-infected with the two isolates displayed similar external clinical signs with the naturally diseased fish. Hemolytic assays revealed β-hemolysis in A. veronii and γ-hemolysis in C. braakii. These findings indicate that both A. veronii and C. braakii contribute to disease outbreaks in yellow catfish.