Lactic acid bacteria (LAB) are widely regarded as safe for use and have long been consumed in various fermented
foods. Selenium offers substantial potential benefits, including immune system support, fertility enhancement, and
antimicrobial and anticancer activity. This study investigated the inhibitory potential of biogenic selenium nanoparticles (SeNPs) prepared by using Lactobacillus casei supernatant. Selenium nanoparticles were biosynthesized using L. casei and subsequently characterized through visual observation, UV–visible spectrophotometry, Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) analysis, and X-ray Diffraction (XRD). The antimicrobial potential of the Lactobacillus casei-mediated selenium nanoparticles (LC-SeNPs) was evaluated using the agar well diffusion technique. A noticeable colour transition from light yellow to ruby red was observed, indicating nanoparticle
formation. The absorption spectrum of LC-SeNPs extended between 250 and 450 nm, with a prominent surface
plasmon resonance peak recorded at 350 nm. The FTIR absorption peaks were from 3626.00 cm-1 to 460.73 cm-1
indicating hydroxyl, ester aldehyde, amine, phenol, and ether groups, were responsible for effective bio-reduction and
stabilization of the nanoparticles. The SeNPs were observed to be spherical in shape and occurred in aggregated forms,
with particle sizes ranging from 20 to 100 nm. XRD confirmed their crystallographic nature. E. coli (25 mm) and S.
pneumoniae (17 mm) showed susceptibility, demonstrating LC-SeNPs' effective antimicrobial activity. These L.
casei-synthesized SeNPs are eco-friendly, non-toxic and has significant medical application.