Lactobacillus species mediated synthesis of silver nanoparticles and their antibacterial activity against opportunistic pathogens in vitro

Introduction: Application of lactic acid bacteria for synthesis of silver (AG) nanoparticles (NPs) could be a good ecological friendly alternative to chemical and physical methods. The objective of this study was to investigate the biosynthesis of silver NPs using Lactobacillus strains and to compare their monosaccharide composition of capsular exopolysaccharides and the antibacterial activity of synthesized nanoparticles. Methods: The washed cells of 22 Lactobacillus strains were used for in vitro silver nanoparticle biosynthesis from silver nitrate solution. The NPs formation was confirmed by UV-visible spectroscopy and transmission electron microscopy (TEM) analysis. TEM micrographs were used for the evaluation of NPs size. The monosaccharide composition of capsular exopolysaccharides was determined using GC/ MS analysis. The antimicrobial activity was determined by agar well diffusion assay. Results: The capsular layers of Lactobacillus strains contained heteropolysaccharides that were composed mostly of glucose, mannose, galactose and rhamnose in a different molar ratio. It was found that Ag NPs with large size (30.65 +/- 5.81 nm) obtained from L. acidophilus 58p were more active against S. epidermidis, E. coli, K. pneumonia, S. flexneri and S. sonnei compared with Ag NPs from L. plantarum 92T (19.92 +/- 3.4 nm). Conclusion: The size and antibacterial activities of Ag NPs were strain-dependent and such characteristics may be due to the capsular biopolymer composition of Lactobacillus strains used for Ag NPs synthesis.