Plant synthesis of silver nanoparticles for the pathogen inhibition of Gram-positive and Gram-negative bacteria

It is well known that both silver nanoparticles (AgNPs) possess broad spectrum antibacterial properties, having a multimodal action that leads to cell death. This work focuses on the preparation and characterization of AgNPs via the green synthesis route, assessing their effectiveness towards the inhibition of Gram-positive and Gram-negative bacteria, namely Escherichia Coli, Staphylococcus Aureus , and Pseudomonas Aeruginosa . In particular, after preparation, the AgNPs are characterized using scanning electron microscopy (SEM-EDS), X-ray diffraction (XRD) as well as dynamic light scattering (DLS), showing that the majority of nanoparticles are in the 20nm range with spherical or pseudo-spherical shapes. Clinical isolates of Escherichia coli , Staphylococcus aureus , and Pseudomonas aeruginosa were subsequently exposed to varying concentrations of AgNPs (0.25–1.5 mg/ml) and incubated at 37 °C for 24 h to assess their antimicrobial activity. The results showed that the highest antibacterial activity was against the bacterium Staphylococcus aureus , while Pseudomonas aeruginosa was the least affected. AgNPs with an average diameter of approximately 20 nm are regarded as optimal for antibacterial applications, being overall less toxic to mammalian cells and the environment compared to AgNP sizes of <10 nm. Furthermore, a comparison of zones of inhibition (ZOI) resulting from the use of AgNPs is compared to those of graphene oxide (GO) nanosheets with varying oxygen content from literature, showing that while using the same protocols, AgNPs outperform GO across all tested strains and concentrations.