Bioactive Silver Nanoparticles Synthesized Using Endophytic Bacillus subtilis CG1 and Their Antimicrobial and Antibiofilm Potential Against Drug-Resistant Pathogens.

Background/Objectives: The study addresses the global health challenge posed by multidrug-resistant (MDR) pathogens, highlighting the urgent need for alternative antimicrobial solutions. This study investigated the in vitro antimicrobial and antibiofilm potential of endophytic mediated-synthesized silver nanoparticles (AgNPs). Methods: An endophytic bacterium was isolated from the medicinal plant Commiphora gileadensis in Saudi Arabia and identified as Bacillus subtilis CG1 through 16S rRNA gene sequencing. The bacterium was utilized for the green synthesis of AgNPs, as confirmed by Ultraviolet-visible (UV-Vis) spectroscopy. AgNPs characterization was done using Fourier-transform infrared (FTIR) spectroscopy, Transmission and scanning electron microscopy (TEM and SEM), energy-dispersive X-ray spectroscopy (EDX), and dynamic light scattering (DLS). The antimicrobial efficacy of the fabricated AgNPs was tested against eight clinically relevant pathogens using standard in vitro assays such as the agar disk diffusion method, minimum inhibitory concentration (MIC), minimum bactericidal and fungicidal concentrations (MBC and MFC). Additionally, AgNPs were tested for antibiofilm activity against P. aeruginosa and S. epidermidis. Tested pathogens included Methicillin-Resistant Staphylococcus aureus (MRSA), Staphylococcus epidermidis, Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Candida auris, Candida albicans, and Candida tropicalis. The antibiofilm efficacy was tested using the Crystal violet assay. Results: UV-Vis spectroscopy confirmed AgNP formation with a characteristic absorption peak at 412 nm. FTIR analysis identified the presence of hydroxyl, nitrile, and alkyne functional groups, which are involved in nanoparticle reduction and stabilization. TEM and SEM revealed predominantly spherical AgNPs with sizes ranging from 17 to 72 nm, while EDX confirmed silver as the major elemental component. DLS analysis showed a Z-average particle size of 113.9 ± 67.75 nm and a zeta potential of -24.2 mV. The synthesized AgNPs exhibited concentration-dependent antimicrobial activity, producing inhibition zones of 10-20 mm at 240 µg/mL. MIC values ranged from 6.25 to 25 µg/mL, whereas MBC and MFC values ranged from 6.25 to 50 µg/mL and 25 to 100 µg/mL, respectively. Moreover, bacterial growth kinetics analysis demonstrated a concentration-dependent inhibition of growth by AgNPs at MIC and sub-MIC concentrations. Additionally, AgNPs demonstrated significant antibiofilm activity against P. aeruginosa and S. epidermidis.Conclusions: Overall, B. subtilis CG1-mediated AgNPs exhibited promising physicochemical properties and antimicrobial and antibiofilm activities, suggesting their potential as alternatives for combating resistant and biofilm-associated infections.