嗜麦芽寡养单胞菌胞外产物对斑点叉尾损伤的病理学研究

PATHOLOGICAL STUDIES ON CHANNEL CATFISH INDUCED BY EXTRACELLULAR PRODUCTS OF STENOTROPHOMONAS MALTOPHILIA

采用硫酸铵盐析从嗜麦芽假单胞菌培养物中提取其胞外产物,通过腹腔注射方式,进行了嗜麦芽假单胞菌(P.maltophilia)胞外产物对斑点叉尾(Ictalurus punctatus)损伤的系统病理学研究。结果表明,嗜麦芽假单胞菌胞外产物具有较强的毒力,对斑点叉尾(42.5±4.4)g的半致死剂量(LD50)为3.21mg蛋白/kg体重。病鱼出现神经症状,腹部和下颌充血、出血,腹部膨大,腹腔内充满大量淡黄色或带血的腹水,胃肠道黏膜充血、出血,肠套叠,肝、脾、肾肿大等临床病变。组织学病变主要表现为全身多组织、器官水肿,出血、变性、坏死以及炎症反应,特别是脑、骨骼肌、肝、脾、肾和胃肠道的损伤较为严重。超微结构观察发现病鱼肝、脾、肾和骨骼肌等器官的细胞的超微结构均有较为严重的破坏,线粒体肿胀,嵴断裂或溶解消失,呈空囊状,内质网扩张,细胞核变形,染色质溶解或固缩;研究中还发现嗜麦芽假单胞菌胞外产物可致淋巴细胞凋亡,脾和肾间质内淋巴细胞均表现为细胞核染色质浓缩边移,或核固缩成一个或数个团块凝聚在核膜周边,形成凋亡小体。

Stenotrophomonas maltophilia is an aerobic, nonfermentative, gram-negative bacterium ubiquitous in nature. It is a causative agent of the channel catfish （Ictalurus punctatus）. S. maltophilia infection is capable of causing high morbidities and mortalities in channel catfish, it is considered to be the most significant factor affecting commercial catfish aquaculture. A stain of S. maltophilia, isolated from the diseased channel catfish in a fish farm in Sichuan Prov-ince, was identified by morphology, physiological and biochemical characteristics analysis, and 16srRNA gene sequence detection. In order to understand S. maltophilia pathogenetic mechanism, the pathology of channel catfish induced by extracellular products （ECPs） of S. maltophilia was studied. The ECPs of S. maltophilia was extracted with ammonium sulfate precipitation, and challenged with channel catfish by the intraperitoneal （i. p.） route. The test was conducted for 7d, with doses of 1.42mg/kg, 1.85 mg/kg, 2.40 mg/kg, 3.13 mg/kg, 4.07 mg/kg and 5.29 mg/kg body weight, respec-tively. The mortality was respectively 0.00%, 10.00%, 20.00%, 40.00%, 70.00% and 100.00%. The results showed that the ECPs of S. maltophilia had quite strong virulence to channel catfish, and the LD50 to channel catfish was about 3.21mg-protein/kg body weight. Challenged fish showed neurosis, congestion and hemorrhage in the abdomen and lower jaw, distension of abdomen, yellowish or flooded fluid in the peritoneal cavity, congestion and hemorrhage in mucosa of gastrointestinal tract, intussusception in rectum, swelling in liver, kidney and spleen. Histopathological examination showed edema, hemorrhage, degeneration, necrosis and the inflammatory response in many organs; especially, serious lesions in brain, kidney, liver, gastrointestinal tract, spleen and skeletal muscle. The kidney showed edema, degeneration and necrosis in renal tubular epithelia, and with macrophages and neutrophi infiltration in the interstitial substance. Edema, vacuolar degeneration and necrosis were seen in liver cells. Epithelia of the gastrointestinal tract underwent degeneration and necrosis, while the lamina propria and tunica submucosa showed edema and infiltration with lymphocytes, macrophages and neutrophi. Congestion, hemorrhage and reduction of the lymphocytes were ob-served in spleen. Degeneration, necrosis and inflammatory cells infiltration appeared in skeletal muscles. Edema and neuronal shrinkage appeared in brain. Concentrated nucleolus, swelling mitochondria and dilatated endoplasmic reticu-lum were ultrastructurally observed in visceral organ （liver, spleen and kidney） cells. The lymphocytes of spleen and kidney underwent chromatin condensation and margination, forming the apoptotic bodies. These results indicated that the ECPs was an important pathogenic factor of S. maltophilia, which could induce apoptosis in the lymphocytes, miltiple organ damnification and death.