Isolation, identification and drug sensitivity of Edwardsiella tarda from Pelteobagrus fulvidraco

To determine the cause of Pelteobagrus fulvidraco's disease outbreak on a breeding farm in Rongchang, Chongqing, we isolated a dominant bacterium from the liver of diseased fish and named it as ET230818. Based on morphological observation, physiological and biochemical identification and through the analysis of 16S rRNA, gyrB, rpoB and cpn60 gene sequences, we confirmed ET230818 as Edwardsiella tarda. The artificial infection experiment finds out the median lethal dose (LD₅₀) of strain ET230818 to P. fulvidraco Average body mass of (17.5±2.5) g, average body length of (15.0±1.2) cm was 2.51×10⁶ CFU·mL⁻¹. Virulence gene test results show that the strain carried sodB, katB, esrB, gadB, fimA, citC and mukF virulence genes. Histopathological observation reveals that the infected P. fulvidraco had disordered and swollen hepatocytes, with hemosiderin concentration in spleen tissue; the renal tubules contracted and narrowed, and the structure of the renal tubules became loose and atrophied; the arrangement of intestinal mucosal cells was disorderly, necrotic and shedding; the intestinal villi disintegrated; and the branchial lamella was obviously deformed and bent. The antimicrobial susceptibility test and Chinese herbal antibacterial experiment show that strain ET230818 was highly sensitive to 11 western medicines such as Doxycycline and Enrofloxacin, and Chinese herbs like Syzygium aromaticum and Galla chinensis had strong bacteriostatic effects on E. tarda. This study can provide references for the diagnosis and drug control of E. tarda from P. fulvidraco.