Biosynthesis and characterisation of antimicrobial silver nanoparticles from a selection of fever-reducing medicinal plants of South Africa

Most developing countries, including South Africa, depend strongly on traditional medicine for a therapeutic outcome and have therefore employed numerous medicinal plants to treat fevers. Therefore, it is imperative that fever-reducing medicinal plants are investigated to establish their efficacy and to determine their potential as sources of new antimicrobials. The incorporation of nanotechnology in antimicrobial research with reference to medicinal plants is a growing domain. The interest in silver nanoparticles (AgNPs) encompasses the tested hypothesis that the chemical combination of silver with medicinal plant extracts results in nanoparticles with enhanced antimicrobial properties in comparison with plant extracts alone. This study investigated the antimicrobial properties from 10 medicinal plants of commercial significance used traditionally for the treatment of fever in South Africa and their potential for enhanced antimicrobial efficacy when incorporated within AgNPs. Plant extracts and AgNPs were tested against fever-related pathogens, i.e., two Gram-positive pathogens; Listeria monocytogenes (ATCC 19111) and Enterococcus faecalis (ATCC 29212) as well as two Gram-negative pathogens; Klebsiella pneumoniae (ATCC 13883) and Acinetobacter baumannii (ATCC 19606) using the broth microdilution method. Chemical characterisation of AgNPs included Ultraviolet-visible (UV-Vis) spectroscopy, dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), and Transmission electron microscopy (TEM). The toxicity profiles of the AgNPs were evaluated using the brine-shrimp lethality assay (BSLA). Silver nanoparticles of both Eucomis autumnalis and Sclerocarya birrea display a dramatic increase in antimicrobial activity against the four test pathogens compared to the respective aqueous plant extracts. The greatest difference in antimicrobial activity was observed against E. faecalis where an increase in antimicrobial activity of at least 50-fold when E. autumnalis aqueous sample were compared with the AgNP-counterparts. Cytotoxicity of both AgNP samples from the BSLA emerged at less than 50% mortality, the results obtained in this study justify the use of selected fever-reducing plant extracts with the biosynthesis of AgNPs as promising antibacterial agents with low toxicity.