Establishment of a bacterial infection model in silkworm （Bombyx mori） and synergistic pathogenicity characteristics of different bacterial strains

【Objective】 This study aimed to establish a bacterial infection model in silkworm（Bombyx mori） to investigate the synergistic pathogenicity of different bacteria, to clarify the pathogenesis and epidemiological characteristics of bacterial diseases of silkworm in Yunnan sericulture area, providing scientific guidance for the prevention and control of bacterial diseases during silkworm rearing. 【Method】 A bacterial infection model in B. mori was constructed based on three factors: instar, infection concentration and infection method. Five strains of intestinal pathogens Mammaliicoccus sciuri（Staphylococcus sp.）, Serratia marcescens, Citrobacter freundii, Klebsiella variicola（Klebsiella sp.）, and Pseudomonas soli（Pseudomonas sp.） were isolated from naturally infected silkworm in Yunnan. The pathogenicity of single infections with each strain and 17 synergistic infection combinations were conducted. 【Result】 The established bacterial infection model in silkworm showed that silkworm larvae infected through population feeding of bacteria had a lower survival rate than those infected individually. An infection concentration of 5.0 OD/mL was recommended for second instar larvae, and 10.0 OD/mL for fourth and fifth instar larvae. All of the five selected bacterial strains were pathogenic to silkworm. The survival rate of infected silkworm generally decreased with the prolongation of infection time. Notably, infection at early instars had more obvious impact on silkworm leading to a large number of death of the fifth instar silkworm, which was consistent with the occurrence pattern of bacterial diseases in sericulture. The survival rate of silkworm with synergistic infection was generally lower than that of single-strain bacterial infections in both early and late instar infections, especially those synergistic infections involving S. marcescens, which resulted in lower survival rates until the spinning stage. Additionally, the survival rate of summer rearing silkworm was generally higher than that of spring rearing silkworm in both second and fourth instar infections, indicating that the disease resistance of summer rearing silkworm in Yunnan sericulture areas was stronger than that of spring rearing silkworm. 【Conclusion】 The five strains of pathogenic bacteria isolated from naturally infected silkworm in Yunnan show obvious pathogenicity to silkworm, and their pathogenicity is related to the infection instar, infection concentration, different bacterial combinations, and infection season.Overall, the specific performance is that the infection at early instars has a more significant impact on silkworm than that at late instars; synergistic infection is more severe than the single-strain bacterial infection; the survival rate of summer rearing silkworm after infection is higher than that of spring rearing silkworm.