Isolation, identification and antimicrobial susceptibility test of Edwardsiella tarda in Bahaba taipingensis

Since the summer of 2023, there have been cases of outbreak and death in the Bahaba taipingensis Natural Conservation Area in Dongguan City. The symptoms mainly include loss of appetite, reddening and congestion of body surface, protruding eyeballs, opaque and ulcerated corneas, as well as ulceration in snout. Dissection revealed a large amount of ascites in abdominal cavity and enlarged liver with brown color. To determine the cause of the disease, and make prevention and control measures, we collected tissue samples such as heart, liver, spleen, kidney, intestine and gills from the dead fish. A dominant strain was isolated from and named as DG230920. Through a comprehensive analysis of its morphological characteristics, physiological and biochemical properties, in addition to the 16S rRNA gene sequence analysis, the bacteria were identified as Edwardsiella tarda. Pathological histological examination results show obvious inflammatory reactions in heart, liver, spleen, kidney and gills. Hemorrhage and desquamation of epithelial cells were observed in intestine; sparse lymphocytes and necrosis of endothelial cells were found in the spleen; tubular degeneration and necrosis were observed in the kidney. Gills showed shedding and hemorrhage of gill filaments. Drug sensitivity analysis of the isolated strain revealed high sensitivity to ciprofloxacin and levofloxacin, with a minimum inhibitory concentration of only ≤0.25 mg·mL⁻¹. The next most effective drug was imipenem, with a minimum inhibitory concentration of ≤0.5 mg·mL⁻¹. The results indicate that the cause of B. taipingensis disease might be an infection with E. tarda, and the isolated strain exhibited stable genetic sequences and sensitivity to clinically used antibiotics. The study suggests that prevention and control measures should be strengthened against E. tarda infection in B. taipingensis, and sensitive drugs should be used for treatment.