Melatonin-enhanced biosynthesis of antimicrobial AgNPs by improving the phytochemical reducing potential of a callus culture of Ocimum basilicum L. var. thyrsiflora

Herein, we describe the green synthesis of silver nanoparticles (AgNPs) using Ocimum basilicum L. var. thyrsiflora leaf derived callus extracts formed in response to 9.0 μM of thidiazuron (TDZ) alone and a combination of 9.0 μM of TDZ + 15 μM of melatonin (Mel) which act both as reducing and stabilizing agents. Based on the phytochemical analysis, the highest total phenolic and flavonoid content were recorded (44.9 mg g−1 DW and 31.5 mg g−1 DW, respectively) in the callus grown on TDZ (9.0 μM) + Mel (15 μM) compared to the TDZ alone (29.8 mg g−1 DW and 17.6 mg g−1 DW, respectively) induced callus. The biosynthesized AgNPs were confirmed by analyzing the excitation of surface plasmon resonance using ultraviolet-visible spectroscopy at 420–430 nm. The synthesized AgNPs were characterized via X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy, which indicate that more potent antibacterial and antileishmanial activity is displayed by the Mel plus TDZ callus extract mediated AgNPs than the TDZ-only callus extract mediated AgNPs against multiple drug resistant human pathogens and Leishmania major. Determination of the antileishmanial efficacy of AgNPs is a major step towards the development of new compounds containing nanoparticles for leishmaniasis treatment.