Green Synthesis of Silver Nanoparticles with Antimicrobial Properties Using Phyla dulcis Plant Extract

Foodborne illnesses caused by the consumption of food contaminated with foodborne pathogens at infectious doses are becoming a common health issue throughout the world. Metal nanoparticles with potential antimicrobial properties are an area that can be explored to discover novel antimicrobial agents. The traditional synthesis methods of metal nanoparticles involve the use of toxic chemicals and the generation of harmful byproducts. In this study, a greener method to synthesize silver nanoparticles (AgNPs) with potential antimicrobial properties was investigated. The aqueous extract of the medicinal plant Phyla dulcis Trev. (verbenaceae) was used as the reducing and stabilizing agent to synthesize AgNPs using microwave irradiation. The formation of AgNPs was confirmed using ultraviolet-visible spectroscopy by the appearance of characteristic surface plasmon resonance peaks in the 430-440 nm wavelength range. The size and stability of the AgNPs were studied using Zetasizer nano-series for 5 weeks after synthesis. The average particle size remained between 63 and 76 nm during the first 4 weeks and increased to 114 nm in the fifth week showing possible aggregation after the fourth week. The zeta potential remained between -20 and -24 mV throughout the 5 weeks showing relatively good stability. Scanning electron microscopy/energy dispersive X-ray spectroscopy showed the association of phytoconstituents with the AgNPs. X-ray photoelectron spectroscopy analysis confirmed the formation of metallic nanoparticles starting from silver nitrate. Finally, the AgNPs were tested to be effective against Escherichia coli O157:H7 (ATCC 43888), Salmonella Typhimurium (novobiocin and nalidixic acid resistant strain), Listeria monocytogenes (4b; ATCC 19115), and Staphylococcus aureus (ATCC 6538) strains, which are known to be common foodborne pathogens.