Culture Method-Dependent Variation in the Sensitivity of Escherichia coli to Silver Nanoparticles

Preparation of various metal nanoparticles using plant extracts has been well studied in recent years. In this study, we found that nanoparticles synthesized using the extracts of the inflorescence of Cocos nucifera exhibited differential inhibitory activity against Escherichia coli depending on the nature of the bacterial culture source. Incorporation of silver nanoparticles (Ag-NPs) into the nutrient broth culture of E. coli resulted in poor inhibitory activity. However, when the silver nanoparticles are added to nutrient agar plates used for culture of E. coli, effective inhibition was observed. Additionally, E. coli in broth culture resisted the inhibitory effects of Ag-NPs by forming aggregates of bacterial cells. The aggregates then generated a protective zone around the colonies to prevent the entry of Ag-NPs, and the bacterial cells multiplied without inhibition by the Ag-NPs. These differential effects of Ag-NPs on E. coli culture grown in nutrient broth and on nutrient agar plates indicated that E. coli in broth culture formed aggregates of cells to develop a biofilm for protection against toxins probably via the quorum sensing mechanism.