Studies On The Identification And Virulence Of Photobacterium Damselae Isolated From Pseudosciaen Crocea

The pathogenicity of pathogen to its host is mainly determined by the relationship between the host, the environment and itself, its pathogenic process includes adhesion, invasion,proliferation in the body and producing toxins. Pathogenic reaction is mainly caused by its invasion and proliferation accompaning the host cells and tissue damage, as well as the interference of the bacterial extracellular products and the destruction the body’s normal metabolism or function. Because the extracellular product is one of the important factors for determining the virulence of pathogenic bacteria, the intracellular survival plays an important role in the pathogenic processes of bacteria, the adhesion to the surface of the host cell is the precedent condition of pathogenicity, the comprehensive understanding of bacterial pathogenicity can be acquired with the study of the extracellular products, the process of intracellular survival and adhesion processes. Our laboratory had isolated a strain of Photobacterium damselae from the nidus of Pseudosciaen crocea for the first time. In order to lay the foundation of the disease control, a series of experiments had been done with the main pathogenic bacteria Photobacterium damselae, focusing in the separation of the bacteria,identification, also in the pathogenicity.23 bacteria had been isolated from pathogenic P. crocea, and the initial artificial infection experimental results showed that strain NPL1006 was the most pathogenic strain. The results of artificial infection experiment showed that the LD50 of strain NPL1006 to P. crocea was7.96×103 CFU/g body weight and strain NPL1006 had strong pathogenicity. The results also showed that the main symptom of the death fish were accordant with the sick ones, and one of the dominant strains isolated from dead fish was as same as strain NPL1006. Strain NPL1006 was identified as Photobacterium damselae by Time of Flight Mass Spectrometer and the 16 S rDNA sequencing analysis. Susceptibility experiments showed that strain NPL1006 was highly sensitive to 9 drugs such as Cefazolin, Cefalotin, Ceftazidime Pentahydrate, Gentamicin,kanamycin, Norfloxacin, Ofloxacin for Injection, Enoxacin and Polymyxin B Sulfate.The glass paper flat-covering method was applied to obtain the extracellular products of strain NPL1006. The results showed that the extracellular products were found to exhibit protease, gelatinase and amylase activities. It also had a strong hemolytic effect on sheep redblood cells and ones of P. crocea. Then, we had studied that the survival rate of strain NPL1006 in macrophages was 23.51%. The adhesion test of strain NPL1006 to various mucus of the P.crocea showed that the adhesive quantity increased with incubating time and reached maximum in the pH of 6. A higher adhesive quantity was achieved at acid condition. Adhesion of strain NPL1006 to the mucus of P. crocea was obviously better in a higher Na+ density. The concentrations of Ca2+ can significantly enhance the adhesion of P. crocea to the epidermal mucus, gills mucus and intestinal mucus.