Loss of outer membrane integrity in Gram-negative bacteria by silver nanoparticles loaded with Camellia sinensis leaf phytochemicals: plausible mechanism of bacterial cell disintegration

Abstract

New generation antimicrobial and smart drugs are the needs of the present era in fighting microbial infection and various chronic diseases. Nowadays nanoparticles (NPs) are widely applied in biomedical fields by virtue of their surface modification, which enhances both target selectivity and function. This study is a continuation of our earlier study that demonstrated antimicrobial property of NPs against both Gram-positive and Gram-negative organisms (Goswami et al, 2015). Silver NPs were synthesized using tea leaves (Camellia sinensis) decoction and were characterized using UV-vis spectrophotometry, transmission electron microscopy (TEM) and Fourier-transformed infrared spectroscopy (FTIR). The silver NPs were stable at various environmental conditions. The stability of the particles may be due to various phytochemicals of tea that were bound to the surface of reduced silver ions as a capping agent. The antimicrobial activity of NP was investigated against three Gram-negative pathogenic bacteria (Shigella dysentriae, Salmonella infestis and Vibrio parahaemolyticus). The outer membrane of Gram-negative bacteria is a lipopolysaccharide (LPS) in nature and provides protection from various stress conditions and antibiotics. But a silver NP destroys its membrane integrity and thus helps in cell killing. Spectral changes confirmed NP interaction with hydrophobic moiety of LPS. Minimum inhibitory concentrations for S. dysentriae, S. infestis and V. parahaemolyticus were 3.75, 5.25 and 5.25 μg ml ⁻¹, respectively. Inhibition of biofilm formation was significant with the three bacterial strains. Cytoplasmic leakage from each bacterial strain was also demonstrated on account of NP treatment. The particles demonstrated good biocompatibility. No damage of human buccal mucosal cells was recorded even at concentration of 10 mg ml ⁻¹. Thus, silver NPs would be potential oral therapeutic molecules against Gram-negative bacteria.