Eco-friendly Biosynthesis of Silver Nanoparticles Using Elephantorrhiza elephantina: Characterization and Antimicrobial Activity against Mycobacterium tuberculosis Surrogate Models

Abstract

The emergence of multidrug-resistant (MDR) and extensively-drug resistant (XDR) Mycobacterium tuberculosis (MTB) strains have scampered efforts to arrest the tuberculosis (TB) pandemic due to treatment failures based on current drugs. The need for novel therapeutics has never been more urgent. Research is leaning towards alternative composite antimicrobials such as silver nanoparticles (AgNPs). Elephantorrhiza elephantina (E. elephantina) is an indigenous Southern African rhizomatous plant which has been used for eons in African traditional medicine for the management of TB. The purpose of this study was to biosynthesize and characterize AgNps using E. elephantina, and to evaluate their antimicrobial effectiveness against surrogate MTB species. The study also sought to screen for the participating secondary metabolites in E. elephantina acting as bio reducing, capping and stabilising agents; and to determine the toxicity of the lyophilised root extract in vivo. Qualitative phytochemical screening techniques confirmed the presence of bioactive secondary metabolites with functional groups capable of mediating in the biosynthesis of AgNPs. UV-Vis spectrometry established the identity of the nanostructures as AgNPs. Transmission electron microscopy and dynamic light scattering confirmed the AgNPs to have an average size of 44 nm, with spherical and cubic morphologies. The biosynthesised AgNPs were potent inhibitors of the model MTB species with an MIC of 19.53 µg/mL, and inhibition zones of 20mm and 18mm against Mycolicibacterium aurum and Mycolicibacterium smegmatis respectively at their MICs. Oral toxicity evaluations based on OECD guideline 425 determined the LD50 of the lyophilised E. elephantina root extract to be safe above 4000mg/kg body weight. The study concluded that biosynthesized AgNPs from E. elephantina root extract, using a one-pot green-synthesis technique, were both safe and effective, offering a promising alternative therapeutic strategy for combating multidrug-resistant TB.