Bacterial disease occurrence and pathogen isolation and identification of BmNPV pressure selected strains in silkworm(Bombyx mori)

【Objective】To investigate and identify the causes of bacterial diseases in the Bombyx mori nucleopolyhedroviurs(BmNPV)pressure selection related strains of silkworms,in order to provided a theoretical basis for the prevention and control of BmNPV resistant bacterial diseases in silkworm production.【Method】Under the same batch and feeding conditions,a preliminary investigation was conducted on the occurrence of bacterial diseases in the normal group of BmNPV pressure selective strain P50Nn(n indicated generation)and four female lines Su N,Ju N,Ming N and Hu N of anti-BmNPV breeds. The intestinal bacteria of the diseased silkworms were isolated and cultured. Then carried out 16S rDNA molecular identification,morphological identification such as Gram staining and scanning electron microscopy, and 20 biochemical identifications including sugar fermentation and indole test.【Result】Bacterial diseases caused the cocooning rate of P50N3 in the BmNPV pressure selection strain of silkworms fed with BmNPV for the third generation to be 47.63%,the cocooning rate of JU N in the mother strain of BmNPV resistant variety that fed with BmNPV for multiple generations was the lowest at 34.72%,while the cocooning rate of P50 in the control strain without BmNPV was 96.15%, the cocoon formation rates of P50N3 and JU N were extremely significantly different from those of P50(P<0.01). The intestinal bacteria of diseased silkworms had been separated and purified for multiple times,and there were many spherical bacteria observed under the microscope. After molecular identification,seven types of bacteria were obtained,including Enterococcus,Staphylococcus,Citrobacter,Serratia,Klebsiella,Pseudomonas and Enterobacter,among which Staphylococcus accounted for the highest proportion of 30.00%,followed by Enterococcus and Serratia,both accounting for 16.67%. The 8 different bacterial strains with the highest consistency in molecular identification were selected for further morphological and biochemical identification,which were consistent with the results of molecular identification.【Conclusion】The bacterial diseases of BmNPV pressure selection strains are mainly caused by Staphylococcus,Enterococcus and various Enterobacteriaceae bacteria,and are more serious in the early stage of BmNPV pressure selection.