Comparison of antibacterial and antibiofilm activities of biologically synthesized silver nanoparticles against several bacterial strains of medical interest

Biological routes have been extensively explored in the synthesis of metallic nanoparticles due to their simplicity and low cost. Among metallic nanoparticles, silver nanoparticles (AgNPs) are widely used in medical applications because of their potent antimicrobial activity. In this work, the ability of the mycelium-free fungus extract produced by Chilean white-root Stereum hirsutum and two plant extracts (green tea and dill) were used in the synthesis of AgNPs. The synthesized nanoparticles were extensively characterized by different techniques. The antibacterial activity of the nanoparticles was demonstrated against the Gram-positive strain Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 3229 (standard CLSI); and the Gram-negative bacterial strains (standard CLSI) Escherichia coli ATCC 25922, Klebsiella pneumoniae ATCC 13803 and the multidrug-resistant Pseudomonas aeruginosa KPC 37. The values of minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were obtained in the range of 1.56–25 µg mL⁻¹. AgNPs synthesized by dill extract showed lower values of MIC and MBC, compared to other nanoparticles. The potent antibiofilm ability of AgNPs was demonstrated, including against the multidrug-resistant P. aeruginosa KPC 37 strain. All synthesized nanoparticles demonstrated antibiofilm activity. AgNPs synthesized by fungus extract demonstrated superior antibiofilm activity, compared to AgNPs synthesized by green tea or dill, at low concentration (1.56 µg mL⁻¹). To our best knowledge, this is the first report to compare the antioxidant and antibacterial effects of AgNPs synthesized by different biological entities (green tea, dill and S. hirsutum) with great importance in the combat of resistant bacteria and biofilms.