一株牙鲆源黏质沙雷氏菌YP1的分离鉴定及致病性分析

黏质沙雷氏菌(Serratia marcescens)是引起人类、动物及植物感染的重要条件致病菌，但其作为鱼类致病菌却鲜有报道。【目的】本研究以从患病牙鲆(Paralichthys olivaceus)病灶处分离的一株黏质沙雷氏菌YP1为研究对象，分析黏质沙雷氏菌对鱼类的致病性及对疾控的影响。【方法】利用形态学、分子生物学及生理生化实验综合鉴定菌株YP1；利用菌株YP1进行人工感染实验、组织病理实验及药敏试验，研究其感染症状、组织病理学、毒力和药物敏感性。【结果】分离自患病牙鲆体表溃疡病灶处的菌株YP1鉴定为黏质沙雷氏菌。感染实验结果显示，牙鲆和斑马鱼的半数致死量(LD₅₀)分别为3.44×10⁷CFU/g和6.28×10⁵CFU/g，除牙鲆外菌株YP1对其他鱼类也具有高致病性；菌株YP1主要导致牙鲆腹水，同时伴有呼吸急促、摄食减弱、脱肛、白便、鳃缺血及多脏器膨大出血等症状，并随着感染时间的延长对脏器损伤呈加重趋势。病理组织切片结果显示，菌株YP1对牙鲆鳃、肠、肝、脾、肾、心均造成损伤。药敏试验结果表明，YP1对左氧氟沙星、诺氟沙星等14种药物敏感；但对氨苄西林、头孢拉定等19种药物具有耐药性。【结论】本研究结果证实了黏质沙雷氏菌是能导致牙鲆腹水病的一种病原菌，同时对其他鱼类也具高致病性，为该菌感染鱼类导致疾病的检测、鉴别和防治提供科学依据。

Identification and pathogenicity characterization of a Serratia marcescens strain YP1 isolated from Paralichthys olivaceus

As a major opportunistic pathogen, Serratia marcescens causes infections in human, animals, and plants, while it is rarely reported as a pathogenic bacterium of fish. Objective] To study the pathogenicity of S. marcescens YP1 isolated from a diseased Japanese flounder (Paralichthys olivaceus) and the adverse effects of the pathogen on the fish disease control. Methods] The morphological characteristics, molecular evidence, and physiochemical properties were combined to identify the strain YP1. Furthermore, the artificial infection experiment, histopathological examination, and drug sensitivity test of the strain YP1 were performed to study its infection symptoms, histopathology, virulence, and drug sensitivity. Results] The strain YP1 isolated from the surface ulcer of a diseased Japanese flounder was identified to be S. marcescens. It had high pathogenicity to both Japanese flounder and zebrafish (Danio rerio), with LD₅₀ being 3.44×10⁷ CFU/g and 6.28×10⁵ CFU/g, respectively. YP1 mainly led to ascites fluid in Japanese flounder, which was accompanied by rapid breathing, reduced food intake, anal prolapse, white stool, gill ischemia, and multiple organ tumefaction and hemorrhage. With the extension of infection time, the organ damage became aggravated. The pathological sections showed that YP1 caused the damage of the gills, gut, liver, spleen, kidney, and heart in Japanese flounder. The results of drug sensitivity test showed that YP1 was sensitive to 14 antibiotics such as levofloxacin and norfloxacin and resistant to 19 antibiotics including ampicillin and cefradine. Conclusion] This study confirmed that S. marcescens was a pathogenic bacterium causing ascites disease of Japanese flounder, and it was also highly pathogenic to other fish. Meanwhile, this study provides a scientific basis for the detection, identification, and prevention of the fish disease caused by S. marcescens.