我国淡水鱼类柱形病病原菌柱状黄杆菌的遗传多样性

为认识我国淡水鱼类烂鳃病的病原以及柱形病在我国的发生情况,实验从发生烂鳃病的病鱼中分离细菌性病原,经过生理生化特性分析以及是否在含托普霉素的Shieh培养基中生长并形成黄色假根状菌落,是否产生降解明胶和硫酸软骨素的酶类等特性的鉴定,并结合16SrDNA序列分析,证实柱状黄杆菌(Flavobacterium columnare)是所分离的烂鳃病的病原。同时,研究也证实20世纪曾经命名为烂鳃(Gill-rot)病病原的鱼害黏球菌(Myxococcus piscicola Lu,Nie & Ko,1975)是柱状黄杆菌的同物异名。利用分离到的16株柱状黄杆菌的16SrDNA序列,以及已经发表的柱状黄杆菌的相关序列,构建了系统发育树,发现柱状黄杆菌的菌株聚成3枝,与柱状黄杆菌的三种基因组型(Genomovar)相对应。其中当时命名为鱼害黏球菌的强毒株G4与分别分离自日本和美国的两株聚为一枝。另外两枝包括的菌株较多,它们中的一些菌株来源于相同的鱼类宿主,如鲤形目的种类;但是,这两枝也包括一些特有的株,如从欧洲和美国的鲑形目鱼类上分离的柱状黄杆菌聚为一枝,这一枝还包括我国曾经命名为鱼害黏球菌的G18弱毒株。从我国隶属于鲈形目的鳜鱼和鲟形目的中华鲟上分离到的柱状黄杆菌则聚为另外一枝。作者认为对不同基因组型菌株的致病性和致病机理的研究将可能从根本上认识鱼类柱形病的流行规律。  

COLUMNARIS DISEASE AND GENETIC DIVERSITY OF ITS BACTERIAL PATHOGEN FLAVOBACTERIUM COLUMNARE IN FRESHWATER FISH IN CHINA

Flavobacterium columnare, the causative agent of columnaris disease infects a wide range of freshwater fish throughout the world, with major symptoms being gill-rot and caudal rot. Gill-rot disease is also a major disease of freshwater fish in China; although its pathogen has been reported, disputes over its identity still exist. In order to understand the pathogen of gill-rot disease and the columnaris disease in China, bacterial pathogens were isolated from gill-rot diseased fish in fish farms in Hubei, Guangdong, Anhui, Sichuan Provinces and in suburbs of Beijing, China, respectively. The physiological and biochemical characters, growth in tobramycin-contained Shieh agar medium, and the formation of rizoids, as well as the ability in binding Congo red and in degrading gelatin and in production of chondroitinase were examined. These characteristics confirmed that the so-called gill-rot disease pathogen was actually the pathogen of columnaris disease, i.e. F. columnare. Furthermore, 16S rDNA sequences were cloned from 16 strains of F. columnare in the present study, and those downloaded from the GenBank database, a phylogenetic tree was built with the identification of three well-supported clades, corresponding clearly to the reported three genomovars of F. columnare. The first clade contains only three strains, including the most virulent strain G4 isolated in 1970s from grass carp, and two others from Japan and USA, respectively. The second and third clades contain many more strains, including those from same species of fish hosts, such as some cyprinid fish. But strains isolated from salmonids are clustered in the second clade, with the inclusion of a low-virulence strain G18 isolated in 1970s from grass carp in China. Strains isolated from the so-called Chinese perch, Siniperca chuatsi and Chinese sturgeon Acipenser sinensis in China are clustered in the third clade. The bacterium, Myxococcus piscicola Lu, Nie & Ko, 1975 named as the pathogen of gill-rot disease of grass carp Ctenopharyngodon idellus should be the synonym of F. columnare in terms of its similarity with F. columnare in above-mentioned characters and also in 16S rDNA. Further investigation should be carried out to examine if these strains in different clades differ in their virulence and then in pathogenicity.