Abstract
The influence of nanomaterials on the ecological environment is becoming an increasingly hot research field, and many researchers are exploring the mechanisms of nanomaterial toxicity on microorganisms. Herein, we studied the effect of two different sizes of nanosilver (10 nm and 50 nm) on the soil nitrogen fixation by the model bacteria Azotobacter vinelandii. Smaller size AgNPs correlated with higher toxicity, which was evident from reduced cell numbers. Flow cytometry analysis further confirmed this finding, which was carried out with the same concentration of 10 mg/L for 12 h, the apoptotic rates were20.23% and 3.14% for 10 nm and 50 nm AgNPs, respectively. Structural damage to cells were obvious under scanning electron microscopy. Nitrogenase activity and gene expression assays revealed that AgNPs could inhibit the nitrogen fixation of A. vinelandii. The presence of AgNPs caused intracellular reactive oxygen species (ROS) production and electron spin resonance further demonstrated that AgNPs generated hydroxyl radicals, and that AgNPs could cause oxidative damage to bacteria. A combination of Ag content distribution assays and transmission electron microscopy indicated that AgNPs were internalized in A. vinelandii cells. Overall, this study suggested that the toxicity of AgNPs was size and concentration dependent, and the mechanism of antibacterial effects was determined to involve damage to cell membranes and production of reactive oxygen species leading to enzyme inactivation, gene down-regulation and death by apoptosis.
Highlights
Nanosilver (AgNPs) is currently the most widely used and fastest growing class of nanomaterials
The aim of this study was to investigate (i) the role of AgNPs size on the growth inhibition of A. vinelandii (ii) the mechanism of AgNPs toxicity on A. vinelandii, which was determined by examining cell morphology and apoptosis, nitrogenase activity, gene expression, reactive oxygen species (ROS) and ▪OH production (iii) the possibility that the AgNPs are internalized in A. vinelandii cells
Several studies have reported that after exposure of bacteria to NPs, cell wall damage and NPs were found in passaged cells; these findings suggested that nanoparticles could be internalized and delivered to newly formed cells as a component of the cytoplasm [72, 73]
Summary
Nanosilver (AgNPs) is currently the most widely used and fastest growing class of nanomaterials. Due to its unique antibacterial properties, it is widely used in traditional industries and emerging fields such as coatings, food, textiles, cosmetics, water purification, medical treatment [1, 2]. There are more than 1,600 consumer goods containing nanomaterials, 4.2-fold higher than 380 goods containing AgNPs in 2010 (http://www.nanotechproject.org). Effect of silver nanoparticles on Azotobacter vinelandii. The large-scale commercial application of AgNPs has created great economic benefits, but there are risks of AgNPs entering the environment through various channels [3]. The biological toxicity and impacts of AgNPs on the environment have been a major focus of recent attention
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