Bombyx mori bidensovirus infection alters the intestinal microflora of fifth instar silkworm (Bombyx mori) larvae

Bacterial diseases can occur as a result of disruption of the intestinal microbial population in the silkworm, Bombyx mori, and are often induced by bidensovirus (BmBDV) infection. We investigated the effects of BmBDV infection on intestinal microbes and immune gene responses in fifth instar silkworm larvae. Midgut contents were collected from BmBDV-infected and uninfected silkworms at 48, 96, and 144 h post-infection (hpi) and the intestinal flora were analyzed using 16S rRNA gene Illumina MiSeq sequencing technology. The abundance of intestinal bacteria differed between BmBDV-infected and uninfected silkworms. There were no significant differences in bacterial diversity at 48 and 96 hpi, but bacterial diversity in infected larvae was lower at 144 hpi compared with that of uninfected larvae. At the phylum level, the ratio of Proteobacteria was higher in infected larvae than in uninfected larvae at 48 and 96 hpi, but was lower after 144 hpi, while the ratio of Firmicutes had increased relative to uninfected silkworms. At the genus level, the ratio of Enterococcus increased gradually in infected silkworms, however, proportion of bacteria genera Incertae sedis were increased at 96 hpi, and the proportion of Lactococcus had decreased at 96 hpi. Principal component analysis showed that the proportion of Enterococcus species present was negatively correlated with most dominant genera. Increases in the abundances of the genera Anderseniella, Simplicispira, Enterococcus and, genera Incertae sedis, were associated with BmBDV infection. Quantitative reverse transcription-polymerase chain reaction indicated that expression levels of genes associated with immune deficiency (IMD), Toll, and JAK/STAT pathways were higher at 144 hpi with BmBDV infection. Enterococcus abundance was higher and was positively correlated with the expression level of spatzle-1, peptidoglycan recognition protein LE, and peptidoglycan recognition protein LB genes, suggesting that an increase in the abundance of Enterococcus leads to activation of the Toll and IMD immune pathways.