Antibacterial Efficacy of Biogenic Silver Nanoparticles Synthesized using Murraya koenigii against Extremophilic Bacteria from Sandstone Environments

Abstract

Background: Extremophilic microorganisms exhibit distinctive physiological and biochemical adaptations that enable them to withstand and thrive under severe environmental conditions. Consequently, these microorganisms constitute promising sources of bioactive compounds with potential applications in biotechnology and pharmaceuticals. The present study investigated the antibacterial efficacy of green-synthesised silver nanoparticles derived from Murraya koenigii (MK-AgNPs) against extremophilic bacterial isolates obtained from sandstone habitats in Keroo Mine, Jodhpur, Rajasthan, India. Methodology: The bacterial isolates Pseudarthrobacter phenanthrenivorans strain psychrophillus KE2A (GenBank accession no. PP237444.1) and Geobacillus kaustophilus strain KE3H (GenBank accession no. PP257971.1) were selected for evaluation of antibacterial activity using the disk diffusion method. MK-AgNPs were green-synthesised using an aqueous leaf extract of Murraya koenigii and characterised by dynamic light scattering (DLS) and zeta potential analysis. MK-AgNP    solutions at concentrations of 20–2.5 mg/mL were tested, with the standard antibiotic gentamicin used as the positive control. Significant differences were observed across the selected concentrations (p < 0.05–0.001). Results: The MK-AgNPs exhibited a surface plasmon resonance peak at 450 nm, spherical morphology (40–55 nm), a Z-average hydrodynamic diameter of 88.03 nm, and a zeta potential of −25.47 mV, indicating moderate colloidal stability. The MK-AgNPs showed significant antibacterial activity against both extremophilic strains. P. phenanthrenivorans KE2A exhibited a concentration-dependent response, with significantly greater activity at 10 and 20 mg/mL than gentamicin (***p < 0.001), whereas G. kaustophilus KE3H showed a dose-independent response, with all tested concentrations showing significantly greater activity than gentamicin (***p < 0.001). Conclusion: The findings highlight the potential of Murraya koenigii-mediated green-synthesised AgNPs as an antibacterial agent against the tested extremophilic bacteria and provide insights into the use of sandstone-associated extremophiles and nanobiotechnology for potential antimicrobial applications.