Ultra-small silver nanoparticles induced ROS activated Toll-pathway against Staphylococcus aureus disease in silkworm model

Abstract The present study investigated the therapeutic action of flavonoids loaded silver nanoparticles (FLV-Ag NPs) on a silkworm, Bombyx mori L., larvae infected by the S. aureus , where an in vivo model system was used. FLV-Ag NPs were synthesized using a Ricinus communis L. leaf extracted flavonoid mixture in various concentrations. The reduction reaction was performed under a simple sunlight exposure condition. In the reduction process, quercetin and kaempferol loadings were also found. They were confirmed by UV–visible, TEM, XRD, XPS, DLS and FTIR spectroscopic techniques. TEM confirmed that the synthesized NPs were monodispersed and with an average size of 5.8 nm ± 0.04. Initially, those synthesized FLV-Ag NPs were tested against S. aureus in in vitro by comparing their potential inhibitory activity with only flavonoids mixture (FLVs) and pure silver nanoparticles (Ag NPs). Furthermore, these FLV-Ag NPs were used to treat S. aureus infected B. mori larvae and the therapeutic efficacy was measured. Our results demonstrate that the FLV-Ag NPs are effective in curing the S. aureus infection through the induced expression of antimicrobial peptide genes, the enhanced oxidative enzyme levels, and the promoted phagocytosis of S. aureus by the haemocytes in silkworm larvae. On the basis of these results, it is proposed that the mechanism of the antimicrobial activity of FLV-Ag NPs involves the NPs-insect fat body cell association and a reactive oxygen species-mediated Toll-pathway.