Eco-Friendly Synthesis of Silver Nanoparticles Using Thermophilic Bacteria (Caldibacillus thermoamylovorans) and Walnut Shells: Promising Antibacterial Agents for Diverse Applications

The preparation of silver nanoparticles (Ag NPs) via an environmentally friendly green synthesis method using both bacterial and plant extracts represents a promising alternative approach from an ecological point of view. Thermophilic bacteria were isolated from the Ilıca hot spring in Erzurum, and the Caldibacillus thermoamylovorans SA1 strain was identified as having the highest lignin peroxidase enzyme activity. Using waste walnut shells in combination with this bacterium, the optimal conditions for lignin peroxidase enzyme production were determined by two different methods (5 g/L shell amount, 96 h incubation time, 140 rpm agitation, 60°C temperature, and pH 8.0). Characterization of the synthesized AgNPs was performed using various analytical techniques, including UV‒Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), which confirmed the successful production of the nanoparticles. Furthermore, the biological efficacy of the synthesized AgNPs was evaluated by assessing their antibacterial activity against pathogenic bacteria, such as Escherichia coli O157:H7, Klebsiella pneumoniae , Staphylococcus aureus , Streptococcus pyogenes , and Bacillus cereus . The AgNPs showed significant efficacy against all five pathogenic bacteria, with the antibacterial potency ranking as follows: B. cereus  >  E. coli  =  S. aureus  >  K. pneumoniae  >  S. pyogenes . In particular, the highest activity was observed against the pathogen B. cereus (15 mm). The broad-spectrum antibacterial properties exhibited by the Ag NPs synthesized in this study offer a promising and sustainable solution for diverse applications in various sectors, including the environmental, agricultural, medical and pharmaceutical fields. In addition, the use of walnut shell, an agricultural waste product, as a substrate for enzyme production not only enhances the value of agricultural byproducts but also ensures the continued use of renewable and sustainable resources.