Pathogenicity of Aeromonas veronii causing mass mortalities of Odontobutis potamophila and its induced host immune response.

Aeromonas veronii is a freshwater bacterium associated with many diseases in aquatic animals. However, few cases of A. veronii infection were reported in Odontobutis potamophila, which has been becoming a promising fish species in China in recent years. In this study, the dominant bacteria were isolated from diseased O. potamophila showing signs of hemorrhage on fins, ulceration on the dorsal and abdomen. The representative isolate Stl3-1was identified as A. veronii based on analysis of its morphological, physiological, and biochemical features, as well as 16S rRNA and gyrB gene sequences. The median lethal dosage (LD50) of the isolate Stl3-1 for O. potamophila was determined as 4.5 × 105 CFU/mL. Histopathological analysis revealed that the isolate Stl3-1caused considerable histological lesions in the fish, including tissue cell degeneration, necrosis, and inflammatory cell infiltrating. Detection of virulence-related genes showed that A. veronii Stl3-1 was positive for exu, ompA, lip, flaH, hlyA, aer, flgM, tapA, act, flgA, gcaT and flgN. Additionally, quantitive real-time PCR (qRT-PCR) was also undertaken to analyses the host defensive response in O. potamophila infected by A. veronii. The immune-related gene expressions in O. potamophila during experimental infection were monitored at different point of time, and the results showed that the expression levels of MHC II, Myd88, TLR, and SOD were significantly up-regulated in liver, gill, spleen, and head kidney. The results revealed that A. veronii was a pathogen causing mass mortalities of O. potamophila and will contribute to better understanding the host defensive response against A. veronii infection.