Antimicrobial activities of biologically synthesized metal nanoparticles: an insight into the mechanism of action.

Increasing antimicrobial resistance is a clinical crisis worldwide. Recent progress in the field of green synthesis has fascinated scientists and researchers to explore its potentials against pathogenic microbes. Bioinspired-metal-based nanoparticles (silver, copper, gold, zinc, etc.) have been reported to be tested against both Gram-positive and Gram-negative bacteria such as B. subtilis, E. coli, Staphylococcus aureus, etc., as well as some pathogenic fungi including A. niger, F. oxysporum, A. fumigatus, etc., and are testified to exhibit inhibitory effects against pathogenic microbes. The possible modes of action of these metal nanoparticles include: (a) excess production of reactive oxygen species inside microbes; (b) disruption of vital enzymes in respiratory chain via damaging microbial plasma membranes; (c) accumulation of metal ions in microbial membranes; (d) electrostatic attraction between metal nanoparticles and microbial cells which disrupt metabolic activities; and (e) inhibition of microbial proteins/enzymes by increased production of H2O2. Although these pathways are interconnected, information on potential mechanism of most of these biogenic nanoparticles is still limited. Further exploration of these mechanisms could help in tackling the burning issue of antibiotics resistance.