Screening Endophytic Fungi of Australian Indigenous Plants for Biosynthesis of Silver Nanoparticles and Evaluation of Their Antibacterial Properties

The increasing emergence of bacterial resistance to conventional antimicrobial agents underscores the urgent need for alternative therapeutics. In this context, the present study investigated the antibacterial potential of silver nanoparticles (AgNPs) biosynthesised from extracellular extracts of four endophytic fungal species isolated from Eucalyptus globulus and Eucalyptus citriodora. The AgNPs were comprehensively characterised using UV–visible spectroscopy, Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and zeta potential analysis. UV–vis spectra confirmed nanoparticle formation through characteristic absorption peaks between 400 and 500 nm. TEM imaging revealed predominantly spherical nanoparticles, with size ranges of 52–98 nm for EG 5 AgNPs, 86–197 nm for EG 6 AgNPs, 56–171 nm for EG 8 AgNPs, and 50–130 nm for EC 2 AgNPs. High negative zeta potential values (−19.3 to −31.7 mV) indicated moderate colloidal stability. FTIR analysis identified functional groups associated with the reduction and stabilisation of AgNPs. All AgNPs samples exhibited notable antibacterial activity against both Staphylococcus aureus and Pseudomonas aeruginosa, and growth–kill curve analysis demonstrated a marked reduction in bacterial proliferation over 24 h. Collectively, these findings highlight the potential of endophytic fungus-derived AgNPs as effective antibacterial agents.