Green Synthesis of Silver Nanoparticles from Carica papaya Leaves for Antimicrobial Photodynamic Therapy against Staphylococcus aureus and Escherichia coli

The escalating global threat of antimicrobial resistance (AMR) has prompted the urgent search for alternative therapeutic strategies to combat multidrug-resistant pathogens. Among the most concerning are Staphylococcus aureus and Escherichia coli, which exhibit increasing resistance to conventional antibiotics. Nanotechnology-based antimicrobial approaches, particularly those utilizing metal nanoparticles and photodynamic therapy, present promising alternatives. This study aimed to evaluate the antimicrobial efficacy of green-synthesized silver nanoparticles (AgNPs) using Carica papaya L. leaf extract, both alone and in combination with photodynamic therapy (aPDT) via blue diode laser irradiation, against S. aureus and E. coli. AgNPs were synthesized through an eco-friendly green route using aqueous papaya leaf extract as a natural bioreductant. The synthesized nanoparticles were characterized using UV-Vis spectroscopy, revealing a characteristic surface plasmon resonance peak between 400 and 50 nm, and average particle size was determined to be 5–6 nm. The antibacterial activity of the AgNPs was assessed using the disc diffusion method at varying concentrations (0.25–1 mM). Photodynamic activation was performed using a 405 nm blue diode laser at different irradiation times (30–120 s). Statistical significance of antibacterial effects was determined using one-way ANOVA. AgNPs exhibited strong antibacterial activity against both S. aureus and E. coli, with the highest inhibition zones observed at 1 mM concentration. Laser irradiation significantly enhanced the antimicrobial effect of AgNPs in a time-dependent manner. The combination of 1 mM AgNPs with 120 s laser exposure achieved a maximum bacterial kill rate of 94.9%. ANOVA analysis confirmed that both nanoparticle concentration and irradiation duration significantly influenced bacterial mortality (p < 0.05). The study demonstrates that green-synthesized AgNPs, particularly when combined with blue diode laser-mediated aPDT, exhibit potent bactericidal effects against clinically relevant, antibiotic-resistant pathogens. This synergistic, eco-friendly approach holds promise as an effective alternative to conventional antibiotics in addressing the growing challenge of AMR.