Limosilactobacillus fermentum-derived silver nanoparticles: biosynthesis, optimization, and biological activities

Abstract

Probiotic bacteria represent valuable sources of bioactive metabolites with diverse biological functions. This study focused on isolation and identification of promising probiotic isolates obtained from fermented dairy products, aiming to employ their capability for biosynthesizing silver nanoparticles (AgNPs) and to assess their biological activities. Among six probiotic examined isolates, isolate HwOs-2 exhibited the most promising characteristics, synthesizing spherical AgNPs ranging from 6 to 23 nm in size, as visualized by high-resolution transmission electron microscopy (HR-TEM). These nanoparticles displayed a negative zeta potential (−7.11 millivolts), effectively preventing aggregation. X-ray diffraction (XRD) analysis confirmed the crystalline nature of the AgNPs, revealing distinct diffraction peaks at specific 2θ angles (38.2°, 44.3°, 64.5°, and 77.4°) corresponding to the (111), (200), (220), and (311) planes of a face-centered cubic lattice. Fourier Transform Infrared Spectroscopy (FTIR) indicated the presence of organic coatings on the AgNPs, including proteins, amino acids, and carboxylic acids, potentially contributing to diverse biological activities. Isolate HwOs-2 was identified as Limosilactobacillus fermentum through Vitek2 automated system and 16 S rDNA partial sequence analysis. Furthermore, optimization of AgNP biosynthesis using response surface methodology (RSM) revealed the significant influence of silver nitrate solution volume, while pH and filtrate volume exhibit negligible effects and incubation time displays a curvature effect on AgNP production. Antibacterial assays against seven bacterial strains, encompassing both gram-positive and gram-negative species, demonstrated substantial antibacterial efficacy, with inhibition zones ranging from 20.3 to 27.6 mm against S. typhi and MRSA, respectively. Additionally, the AgNPs exhibited antitumor activity against Caco-2 and Huh-7 cell lines, with IC50 values of 350.08 and 388.35 µg/mL, respectively, while displaying lower cytotoxicity against normal (VERO) cells (IC50 value = 622.17 µg/mL). These findings underscore the biomedical potential of AgNPs produced by Limosilactobacillus fermentum across a spectrum of applications.