Pathological Mechanism Of Photobacterium Damselae Subsp.damselae Causing Half-smooth Tongue Sole Skin Ulcer Disease

Affected by global climate change trends,the mesophilic bacterium Photobacterium damselae subsp.damselae will become the emerging pathogen of global marine economic fish farming.In this paper,the pathogenicity of the strain to the half-smooth tongue sole(Cynoglossus semilaevis Günther)was studied by artificial infection experiment,pathogenic bacteria susceptibility test and pathological analysis.Through the intraperitoneal injection and non-invasive immersion infection method,the relationship between the bacteria and the skin ulcer disease of the half-smooth tongue sole was explored,and the relationship between the spread of the bacteria and the water environment was explored.Provide scientific basis for clinical diagnosis and drug prevention and treatment for diseases caused by the bacteria,and provide pathological theoretical support for the research of infection path of the bacteria.The research results are as follows:1.The pathogenicity of P.damselae subsp.damselae to the half-smooth tongue sole was studied using the Koch’s postulates.The results were as follows: Six(16 strains)of bacteria were isolated from the half-smooth tongue sole of the skin ulcer disease,and it was confirmed by the resensing experiment that only the P.damselae subsp.damselae could cause the half-smooth tongue sole disease.The strain is highly pathogenic,and the LD50 was 1.21 × 104 CFU ml-1,which can be expressed after injection of different concentrations of bacterial suspension(~106 CFU m L-1,~104 CFU m L-1)with different clinical symptoms(abdominal swelling,skin ulcers).The sensitivity of the bacterium to antimicrobial agents was analyzed by Kirby–Bauer paper diffusion method,and the results were as follows: P.damselae subsp.damselae was insensitive to antimicrobial agents such as quinolones,aminoglycosides,tetracyclines,macrolides,polypeptides and nitrofurans,while it was highly sensitive to cephalosporins,penicillin,chloramphenicol,carbapenem and sulfonamides antimicrobial agents.2.The relationship between the bacterium and the half-smooth tongue sole skin ulcer was further demonstrated by pathological methods.The results were as follows: although there were significant differences in clinical symptoms between the suspension group of the higher concentration bacteria in intraperitoneal injection and the other two groups(the suspension group of lower concentration bacteria and the natural diseased fish),the pathological damage of the organs(brain,heart,gills,liver,intestine,kidneys)and skin surface muscle fibers in vivo was similar,and usually the pathological changes of disease fish in low concentration group are more similar to those of natural diseased fish.Therefore,through the above pathology comparison,we can further prove that P.damselae subsp.damselae was an important pathogen of skin ulcer disease of half-smooth tongue sole.3.The infection pathway of the bacterium and the causes of skin ulcer in the natural environment were analyzed by using non-traumatic immersion infection method and pathological method.The results are as follows: P.damselae subsp.damselae can use seawater as a medium of transmission,and infected with fish,by destroying the gill cell structure of fish into the blood circulation throughout the fish body,when the bacteria into the skin tissue will cause dissolved necrosis of muscle fibers in the muscle layer,when the necrosis of muscle fibers reached a certain degree,can cause the epidermis and dermis of the place to collapse,fall off,and eventually form a "skin ulcer".