Green biosynthesis and antimicrobial activity of silver nanoparticles synthesized by Lactobacillus acidophilus against multidrug-resistant Klebsiella pneumoniae

The rising global issue of antimicrobial resistance requires an immediate investigation of new antibacterial agents. This study examined the green biosynthesis of silver nanoparticles (BS-AgNPs) using the probiotic bacterium L. acidophilus (L. acidophilus) as a sustainable method to combat drug-resistant K. pneumoniae. The MDR strains were screened, and the morphological properties of the resultant nanoparticles were evaluated using FESEM, DLS, and FTIR. The efficiency of bacterial virulence properties was investigated. Antibacterial experiments, including well diffusion, time-kill, and minimum inhibitory concentration (MIC) tests, were conducted. RT-qPCR evaluated the transcription of the mrkA and mrkD genes, which are critical for fimbrial synthesis and biofilm formation. BS-AgNPs were spherical with a median core size of 117 nm and a surface charge of − 1.2 mV. FTIR spectra revealed prominent peaks at 1634 cm⁻¹ (amide I) and 3311 cm⁻¹ (O–H/N–H stretch), confirming the role of L. acidophilus biomolecules as capping and reducing agents. The nanoparticles showed strong antibacterial efficacy, with MIC values of 1.56–3.125 µg/mL and MBC values of 6.25–12.5 µg/mL. Zones of inhibition ranged from 19 to 33 mm across different strains. Time-kill assays showed a rapid, dose-dependent reduction in bacterial load. RT-qPCR analysis indicated significant downregulation of mrkA and mrkD expression (p ≤ 0.05) in treated isolates. These findings highlight BS-AgNPs as a potent and eco-friendly therapeutic agent against virulent, multidrug-resistant K. pneumoniae infections.