ISOLATION, IDENTIFICATION AND PATHOGENICITY OF THE PATHOGENIC BACTERIUM FROM CENTROPRISTIS STRIATA

To discover the causes of sick black seabass, bacterial strain ZS201807 was isolated from the cultured Centropristis striata with the symptoms of white spots in gill and visceral organs. The biochemical and physiological characteristics of the isolated strain were studied by using conventional method, such as API 20 and 16S rDNA gene sequence. The bacteria were identified as Edwardsiella tarda. The artificial infection experiment indicated that the strain was the causative agent of sick Centropristis striata. Histopathological analysis revealed that the spleen and kidney were the main target organs with serious infection, such as a large number of erythrocyte infiltration in the spleen tissue, serious blood stasis, gill filaments capillary dilation, renal tubular cavity stenosis, glomerular enlargement, epithelial cell swelling and cell cavitation. Ultrastructural pathology showed that there was a large accumulation of rod bacteria in the spleen and head kidney tissue of the sick fish. The drug susceptibility test showed that the bacterium was sensitive to 14 kinds of drugs such as ciprofloxacin (per 5 μg), tetracycline (per 30 μg) and enrofloxacin (per 5 μg), and resistant to 13 kinds of drugs such as penicillin (per 10 U), azithromycin (per 15 μg) and amikacin (per 30 μg). It was confirmed that the pathogen of the disease and death of the black seabass was slow Edwardsiella tarda.