Synthesis of Chitosan Stabilized Silver Nanoparticles and Evaluation of the in vitro Antibacterial Activity Against Xanthomonas oryzae pv. oryzae Causing Blight Disease of Rice

Bacterial blight is one of the most devastating rice diseases that cause huge economic loss worldwide. The cause of rice blight is Gram negative bacteria Xanthomonas oryzae pv. oryzae (X. oryzae pv. Oryzae). Since both silver nanoparticles and chitosan have antibacterial, antifungal and growth-stimulating effect, this work has focused on synthesizing chitosan stabilized silver nanoparticles (AgCSs) with small sizes and in vitro evaluating antibacterial activity against X. oryzae pv. oryzae bacteria. AgCSs were chemically synthesized by reducing silver nitrate by borohydride sodium in the presence of chitosan with optimization of the concentration of the reactants. AgCSs were characterized by UV/vis absorption spectra, field emission scanning electronic microscopy (FESEM), ImageJ software, zeta potential measurement, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction. AgCSs have spherical configuration and narrow size distributions with different average sizes from 15 nm to 25 nm depending on the initial concentration of silver nitrate. All AgCSs colloidal systems were stable and exhibited no tendency for coagulation more than 5 months. It was the first time that chitosan-stabilized silver nanoparticles were assessed the in vitro antibacterial activity against bacterial blight VXO_281 strain. The disc diffusion method demonstrated that the smallest size silver nanoparticles (AgCS1) showed high antibacterial effect against the X. oryzae pv. oryzae VXO_281 strain with a concentration of more than 5 µg/mL and the inhibition zone was dose-dependent. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of AgCS1 against X. oryzae pv. oryzae VXO_281 were 2.5 µg/mL and 20 µg/mL, respectively.