Characterization and study of the antibacterial mechanisms of silver nanoparticles prepared with microalgal exopolysaccharides.

Abstract The green synthesis of biomaterials is of significant interest as it enables the safe and sustainable preparation of noble metallic nanoparticles for medical applications. Microalgae polysaccharides have received attention due to their outstanding properties such as biocompatibility, biodegradability and low cost. In addition, due to their variety of remarkable biological and physicochemical properties, polysaccharide-based nanoparticles have advantageous features yet to be explored. The primary objective of the current research was to investigate exopolysaccharides isolated from green microalgae Botryococcus braunii (EPBb) and Chlorella pyrenoidosa (EPCp), as both reducing and stabilizing agents, for the green synthesis of silver nanoparticles (AgNPs). Their antibacterial activity towards Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), and antibiotic-resistant bacteria (methicillin-resistant Staphylococcus aureus) was studied, as well as their cytotoxicity to human dermal fibroblasts. The presently synthesized AgNPs were spherical in shape and exhibited characteristic surface plasmon resonance at 430 nm. The main population had a particle size which ranged between 5 and 15 nm as analyzed by transmission electron micrographs. Zeta potentials averaged −51.81 ± 3.01 mV using EPBb and −12.16 ± 2.41 mV using EPCp. More importantly, AgNPs possessed strong antibacterial activity in a dose-dependent manner, even against drug-resistant bacteria. The enhanced antibacterial activity of these particles is explained due to extensive reactive oxygen species generation and bacterial cell membrane damage. In contrast, such AgNPs were not cytotoxic at the same therapeutic range to fibroblasts (0.5–10.0 μg/mL). In summary, these results showed that polysaccharide-capped AgNPs have a strong potential for numerous medical applications, such as antibacterial agents in pharmaceutical and biomedical areas. Graphical abstract (A) UV/Vis spectra and TEM micrographs of polysaccharide-capped silver nanoparticles, (B) growth curve and effect on the cell morphology of E. coli treated with the silver nanoparticles. Unlabelled Image Highlights • A new and useful application for exopolysaccharides from microalgae Botryococcus braunii and Chlorella pyrenoidosa. • Green and sustainable approach for the synthesis of silver nanoparticles based on the biorefinery from microalgae is highlighted. • Nanoparticles exhibited antibacterial activity towards Gram-positive, Gram-negative, and antibiotic-resistant bacteria. • The killing mechanisms involved in the observed antibacterial activity were investigated. • The polysaccharide capped‑silver nanoparticles were not cytotoxic to fibroblasts at the same therapeutic range. [ABSTRACT FROM AUTHOR]