Green-synthesized silver nanoparticles against Streptococcus mutans: antibacterial activity and transcriptomic insights into planktonic and biofilm states

Dental caries, one of the most common infectious diseases worldwide, are closely associated with Streptococcus mutans ( S. mutans ) biofilms that exhibit strong resistance to conventional antimicrobial agents. Herein, a green synthesis of silver nanoparticles (AgNPs) is reported to use extracellular metabolic products derived from Pseudomonas putida KT2440 as natural reducing, capping, and stabilizing agents for antibacterial therapy. The resulting AgNPs possess nanoscale size, negative surface charge, and excellent colloidal stability. These green AgNPs display potent antibacterial and antibiofilm activities against S. mutans , significantly disrupting bacterial membranes, suppressing acidogenicity, and inducing metabolic dysfunction. Biofilm evaluation further revealed a marked reduction in bacterial and extracellular polysaccharide biomass, indicating the collapse of the three-dimensional biofilm structure. To gain deeper insight into the molecular mechanisms of AgNPs-mediated antibacterial activity, RNA sequencing (RNA-seq) was conducted, revealing significant transcriptional reprogramming associated with the inhibition of metabolic and translational processes, disruption of cell wall homeostasis, suppression of virulence gene expression, and perturbation of carbohydrate metabolism. This study presents an environmentally friendly and effective strategy that bridges green nanotechnology with oral microbiology, offering a sustainable approach for caries prevention and biofilm control.