Reactive oxygen species induced oxidative damage to DNA, lipids, and proteins of antibiotic-resistant bacteria by plant-based silver nanoparticles

This study assesses the mechanism of action of plant-based silver nanoparticles (AgNPs) against antibiotic-resistant bacteria. We compared AgNPs synthesized through Salvia moorcroftiana and Origanum vulgare extracts and their conjugates with the antibiotic Ceftriaxone for their capacity to cause oxidative damage through reactive oxygen species (ROS). We quantified ROS in the cells of two bacterial strains after treating them with all AgNP types and observed that AgNPs were most effective in K. pneumoniae as they resulted in the highest ChS1 count (44,675), while in P. aeruginosa, Cfx-AgNPs induced the highest levels of ROS with ChS1 count of 56,865. DNA analysis showed that both plant-based AgNPs (O-AgNPs = 0.192 and S-AgNPs = 0.152) were most effective in K. pneumoniae and S-AgNPs (abs = 0.174) and O-Cfx-AgNPs (abs = 0.261) in P. aeruginosa. We observed a significant increase in the levels of conjugated dienes (86.4 μM) and malondialdehyde (172.25 nM) in the bacterial strains after treatment with AgNPs, compared to the control (71.65 μM and 18.064 nM, respectively, in K. pneumoniae and P. aeruginosa). These results indicate lipid peroxidation. AgNPs also increased the levels of protein thiols (0.672 nM) compared to the control (0.441 nM) in K. pneumoniae, except for Chem-AgNPs (0.21 nM). These results suggest that plant-based AgNPs are more effective in oxidizing bacterial DNA, protein, and lipids than Chem-AgNPs. Furthermore, protein oxidation varied between AgNPs alone and AgNPs-antibiotic conjugates. The highest levels of protein thiols were found in the samples treated with O-Cfx-AgNPs (0.672 nM and 0.525 nM in K. pneumoniae and P. aeruginosa, respectively). The results demonstrated that AgNPs kill bacteria by altering bacterial macromolecules such as DNA, lipids, and proteins.