Abstract: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.
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