Effect of Biogenic Gold Nanoparticles on Gut Microbiota Composition during Larval-to-Pupal Transition in <i>Bombyx mori</i> L.

This study investigates the effects of biogenic gold nanoparticles (Au NPs), synthesized using an aqueous leaf extract of <i>Morus alba</i>, as a feed supplement on the gut microbiota of <i>Bombyx mori</i>. The synthesized Au NPs displayed a characteristic peak absorbance at 545 nm in the UV-vis spectrum, indicative of surface plasmon resonance (SPR), a phenomenon typical of Au NPs. Fourier-transform infrared (FTIR) spectra revealed the functional groups responsible for the reduction of Au-(III) to Au(0). The crystallinity of the synthesized Au NPs was checked using TEM and XRD analysis. TEM micrographs further exhibited the quasi-spherical, monodispersed, well-scattered nature of the Au NPs with an average particle size of 34.15 ± 9.45 nm. The presence of (111), (200), (220), and (311) planes in Bragg's reflections confirmed the face-centered-cubic (fcc) crystalline nature of elemental gold. The LC-MS/MS study revealed that the presence of two zwitterionic species (mainly betaine) stabilizes the Au NPs by the formation of positive sol micelles. Biogenic Au NPs at 40-60 ppm concentrations significantly improved larval growth, effective rearing rate (ERR), filament length, and cocoon quality without affecting silk fineness. Doses below 40 ppm were biologically ineffective, while concentrations above 60 ppm triggered oxidative stress and cytotoxicity. Similarly, gut microbiota analysis revealed notable compositional shifts at the phylum, class, family, and genus levels. There was a decrease in the relative abundance of the phylum <i>Cyanobacteria</i>, concomitant with a significant enrichment of stress-resilient taxa, including <i>Planctomycetes</i>, <i>Micrococcaceae</i>, <i>Parcubacteria</i>, and <i>Candidatus Adlerbacteria</i>. Predictive functional profiling using PICRUSt indicated enhanced microbial pathways linked to metabolism, stress response, and detoxification. These results suggest that biogenic Au NPs influence host-microbiome interactions, facilitating improved nutrient assimilation and silk production. However, further research is required to evaluate the long-term safety and ecological impact before widespread use in sericulture.