| Since 2008, a serious disease harmed to farm Atlantic salmon with chronic ulcer disease happened in Dujiangyan,Pengshan and other places in Sichuan, the disease was a chronic disease, and high mortality rates.On the surface appeared round, oval or not rules for the pathological features of ulcer has been the Atlantic salmon farming to the local result in serious economic losses. This test of its etiology, pathology, etc. have been studied, and the purpose is to find out the disease pathogens and pathogenesis, for aquaculture production to effectively diagnose and provide a scientific basis for disease prevention and control.By aetiology study, a pathogenic bacterium (DF081019) was isolated from muscular ulcer and liver of naturally diseased Atlantic salmon. After artificial infection experiments as a pathogen of Atlantic salmon ulcer disease. We studied the bacterium's morphology, physiology, physiological and biochemical analysis showed that the bacterium was gram negative, Most of the colonies existenced separate, a small number of bacteria in pairs,the bacterium showed 1～1.7mm size, the surface was smooth, convex and shiny, in the LB medium it grown translucent, no flagella, no capsule. The main biochemical characteristics are summarized as follow:Bacteria ferment glucose oxidase positive, oxidase test positive, 0% NaCL growth. Able to use mannitol, non-athletic, and able to ferment maltose, glucose, acid-positive, arginine positive double-hydrolase, phenylalanine deaminase negative. On the bacteria's drug sensitivity experiments show that bacteria of doxycycline, tetracycline, florfenicol, gentamicin-sensitive, for azithromycin, clarithromycin, Pioneer ADMⅡ, more sensitive to amikacin, pairs of miriocycline, streptomycin, vancomycin, and ampicillin, sulfisoxazole, neomycin, kanamycin, penicillin, carbenicillin were insensitive. A 1490bp 16sRNA complementary fragment of (DF081019) was checked by amplification using bacteria general PCR primer. The sequence classifer in RDP Database suggested that it belonged to Pseudomonas. A phylogenetic tree was constructed by comparing the 16s RNA sequence of the isolated strain(GenBank) with other relative bacteria species in the GenBank databases. The homology between strain (DF081019) and other Pseudomonas fluorescens was 99.2%. according to phylogenetic analysis, the isolated strain(DF081019) was identified as Pseudomonas fluorescens. The toxicity of (DF081019) was studied by means of I.M.,scuffing immerse and immerse test. The results showed that I.M. and scuffing immerse caused similar skin ulcer and death. The histopathological examinationg showed lesions Department of skeletal muscle interstitial edema, hemorrhage, gap widened, muscle fiber degeneration, sarcoplasmic coagulation, was severely stained, some muscle cells in muscle paste was dissolved, interstitial inflammatory cell infiltration; sarcoplasmic dissolved dissolved into an empty grave bubble-like, only remnants of a small amount of basement membrane, some of the regional muscle fiber fracture; liver cell swelling, vacuolar degeneration, necrosis of partial liver cell lysis, nuclei dissolution disappeared; liver sinusoids edema and expansion, there are protein-like substance leaking; Disse space expansion;a congestion occurs; spleen hemorrhage, a large number of lymphocytes disappeared, occupied by red blood cells, a large number of hemosiderin generation; ulcer at the epidermis and dermis layer of skin edema; intestinal mucosal necrosis, intestinal villi broken off in the intestine; brain matrix loose edema.
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