Silver Nanoparticles (AgNPs) from Lysinibacillus sp. Culture Broths: Antibacterial Activity, Mechanism Insights, and Synergy with Classical Antibiotics.

Abstract

Antibiotic-resistant bacteria pose problems for infection prevention and treatment, so developing new procedures or substances against infection is mandatory. Silver nanomaterials are among the more promising antibacterial agents. Herein, we describe the biogenic synthesis of silver nanoparticles (AgNPs) using culture broths from an undescribed species of Lysinibacillus. Culture broths with or without NaCl and from the exponential and stationary growth phases produced four AgNP types. Nanoparticles' shapes were quasi-spherical, with core sizes of 7.5-14.7 nm and hydrodynamic diameters of 48.5-80.2 nm. All the AgNPs contained Ag⁰ crystals and some AgCl ones. Moreover, their coronas presented different proportions of carbohydrates, proteins, and aliphatic compounds. The AgNPs were good antibacterial agents against six bacterial species, three Gram-positive and three Gram-negative, with MICs of 0.3-9.0 µg/mL. Their activity was higher against the Gram-negative bacteria and particularly against Pseudomonas aeruginosa. These AgNPs acted synergistically with several of the fifteen tested antibiotics. Interestingly, AgNP combinations with some of these inhibited the growth of antibiotic-resistant bacteria, as in the case of S. epidermidis for streptomycin and S. aureus for colistin. The ROS production by E. coli and S. aureus when treated with most AgNPs suggested different mechanisms for bacterial killing depending on the AgNP.