Study on the correlation between Flavobacterium columnare infection and gill injury in Largemouth bass (Micropterus salmoides) and the protective effect of Enoxolone.

Columnaris disease, caused by Flavobacterium columnare, has a broad host range and can afflict multiple families and genera of freshwater fish. In addition, our prior research found that Enoxolone emerged as the most promising candidate with a low minimum inhibitory concentration and good cost-effectiveness. However, the therapeutic effect of Enoxolone on Micropterus salmoides infected with F. columnare remains unclear, and its antibacterial efficiency in vivo has yet to be investigated. In this study, we found that the cumulative mortality rate of M. salmoides reached 70 % within 96 h post-infection with 1 × 10 CFU/mL F. columnare, and the gills exhibited clinical signs such as paleness and ischemia, accompanied by characteristic "Clavate Gill" pathological changes. Meanwhile, both the severity of these lesions and the bacterial load showed a significantly upward trend over time. In addition, following Enoxolone treatment, the mortality rate of M. salmoides infected by F. columnare was reduced by 40 %, with alleviated general pathological changes and a significantly lower number of apoptotic positive cells compared to the infected but untreated group. The relative expression of apoptosis and inflammation-related genes showed that the expression levels of Caspase-9, IL-18, TNF-α and NF-κB were significantly upregulated after F. columnare infection. However, the relative expression of Bcl-2 and IL-10 was significantly upregulated after treatment with Enoxolone. In conclusion, there was a positive correlation between the F. columnare load and gill damage in M. salmoides. In addition, Enoxolone has potential therapeutic and protective effects against gill tissue damage caused by F. columnare. Presumably, this efficacy may stem from the drug's ability to inhibit bacteria, thereby reducing gill cell apoptosis and inflammation. These findings provide new perspectives for future research and establish a groundwork for devising preventive and curative strategies against F. columnare infections.