Pathogenicity of fish pathogen Pseudomonas plecoglossicida and preparation of its inactivated vaccine

Many fishes infected with Pseudomonas plecoglossicida generally suffer from "visceral white spot disease" or even die. In this study, a dominant pathogen strain was isolated from the intestinal tract of diseased crucian carp in the Wangcheng Lake area, Changsha, and it was identified as P. plecoglossicida. The selected strain was a new strain named as P. plecoglossicida LQJ06.Strain LQJ06 basically colonized the intestine and poisoned zebrafish as show by fluorescent labelling. Pathological structural analysis of tissue sections indicated that the intestinal tract was seriously damaged, epithelial cells in the intestinal tissue were necrotic, intestinal villi were sloughed, liver cells were vacuolated, nuclei were pyknotic and shifted, and lymphocytes were proliferated in the spleen. P. plecoglossicida LQJ06 strain could invade and proliferate in the grass carp liver cell line L8824, which led to a stress response, including apoptosis. Cell morphology was changed owing to the toxicity of the culture supernatant of the LQJ06 strain, which mainly manifested as aggregation between cells, pyknosisd and slow growth or even death. An inactivated vaccine derived from P. plecoglossicida LQJ06 prepared in this study was safe and nontoxic to grass carp liver cells. Compared with those after oral administration, most of the cellular immune factors were expressed earlier and at a higher level after injection immunization. The intestinal tract and liver from zebrafish mainly expressed the IFN-γ2 and IL-1β genes, respectively, after immunization. The upregulation of these immune-related genes proved that the vaccine could strengthen the immunity of zebrafish, induce inflammation and promote resistance to pathogenic infection. The results of these preliminary tests provide a scientific basis for further research on the prevention and control of P. plecoglossicida, and an essential preliminary basis for the development of an inactivated vaccine against P. plecoglossicida.