Abstract: In order to determine the cause of death about the simulated ecological Pelodiscus sinensis in Chongqing, the pathogenic bacteria named strain PS01 was isolated from the intestinal and pharynx of deseased P. sinensis. The pathogen isolation, artificial infection, physical and chemical characteristics of isolated bacteria, sequence analysis of 16S rRNA and gyrB, drug sensitivity test and pathological section were carried out and investigated. The results showed that artificial infection symptoms with P. sinensis were similar to the natural symptoms. The phylogenetic tree constructed by 16S rRNA and gyrB sequences was the same as C. freundii, with a similarity of 99.7% and 98.7%. Pathological observation showed, there were various degrees of damage such as cytopathic and necrotic and so on. The pathological observation showed that the liver, spleen, kidney, ancreas, and intestinal had different degrees of lesions of the diseased C. freundii, among them, the three tissues of intestine, liver and spleen were the most serious. Most of intestinal villi were necrotic and shed, hepatocytes swell, nucleus vacuoles, mixed distribution of red and white marrow of spleen, mild pancreatic and renal lesions, pancreatic cells and renal tubular epithelial cells swell, no significant change was observed in other tissues. The drug susceptibility test showed that the strain PS01 was highly sensitive to 18 antibacterial drugs, such as doxycycline; and middle sensitive to 3 antibacterial drugs, such as florfeniol; resistant to 18 antibacterial drugs, such as sulfamethoxazole compound. This study proved that the disease of P. sinensis caused by C. freundii in Chongqing, whose half-lethal concentration for Chinese sturgeon of about 30 g was 9.3×10⁴ CPU·g^-1, which had strong pathogenicity and could cause multiple tissue lesions of P. sinensis.

Key words: Pelodiscus sinensis, Citrobacter freundii, drug sensitivity test, pathological section