Green Synthesis of Silver Nanoparticles with Culture Supernatant of a Bacterium Pseudomonas rhodesiae and Their Antibacterial Activity against Soft Rot Pathogen Dickeya dadantii

Abstract

Bacterial stem and root rot disease of sweet potato caused by <i>Dickeya dadantii</i> recently broke out in major sweet potato planting areas in China and calls for effective approaches to control the pathogen and disease. Here, we developed a simple method for green synthesis of silver nanoparticles (AgNPs) using bacterial culture supernatants. AgNPs synthesized with the cell-free culture supernatant of a bacterium <i>Pseudomonas rhodesiae</i> displayed the characteristic surface plasmon resonance peak at 420-430 nm and as nanocrystallites in diameters of 20-100 nm determined by transmission electron microscopy, scanning electron microscopy, and X-ray diffraction spectroscopy. Functional groups associated with proteins in the culture supernatant may reduce silver ions and stabilize AgNPs. The AgNPs showed antibacterial activities against <i>D. dadantii</i> growth, swimming motility, biofilm formation, and maceration of sweet potato tubers whereas the culture supernatant of <i>P. rhodesiae</i> did not. AgNPs (12 µg∙ml^-1) and AgNO₃ (50 µg∙ml^-1) showed close antibacterial activities. The antibacterial activities increased with the increase of AgNP concentrations. The green-synthesized AgNPs can be used to control the soft rot disease by control of pathogen contamination of sweet potato seed tubers.