## 8 Mechanism of Microbial Activity  
Nanoparticles are toxic to human and other living organisms. Due to their nano scale, they are easily exposed to humans and other organisms through ingestion, inhalation and dermal contacts. A large number of publications by various authors is available on the behavior, characterization and toxicological information of nanomaterials [110]. The focus of this research is mainly on manufacturing of commercialized nanomaterials, which are widely applicable, such as fullerene, metal oxides and CNTs. It is important to assess the fate of nanoparticles on the environment when applying them commercially on a larger scale. Ag nanoparticles are capable of interacting with the surface of various bacterial cells. This is especially relevant when dealing with Gram-negative bacteria because accumulation and adhesion of Ag NPs to the surface of bacteria has been observed in numerous studies. By damaging cell membranes, Ag NPs lead to structural changes, making bacteria more permeable [111]. This change is directly related to concentration, shape and size of the nanoparticles [112]. This influence is confirmed by a study using _Escherichia coli_ that affirmed that gaps in the integrity of the player are created by the accumulation of Ag NPs, which increased the permeability leading to cell death of bacteria [113].  
Due to their widespread medical, military and industrial applications, metal oxide nanoparticles, such as zinc oxide (ZnO), silicon dioxide (SiO\({}_{2}\)), aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) and titanium dioxide (TiO\({}_{2}\)), have gained a lot of interest. They affect soil, aquatic organisms and human health upon their release into the environment. So far, the mechanism of toxicity for each nanoparticle is not understood exactly, but various characteristics may result in damage to the exposed organisms. Reactive oxygen species (ROS), such as super oxides (O\({}_{2}^{-}\)), singlet oxygen (\({}_{1}\)O\({}_{2}\)) and free radicals (OH\({}^{-}\)), are generated by nanoparticles that employ various adverse effects on microbes, such as scattered vesicles, disruption of cell wall, enzyme inhibition and protein and sugar membrane leakage leading to slow dissolution and resulting in inhibition of cellular growth and respiration [92; 102]. During some studies, the metal oxide nanoparticles, ZnO, SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), were proven harmful to _Pseudomonas fluorescens_, _ Bacillus subtilis_ and _Escherichia coli_[114]. Significant toxicity was caused by these nanoparticles to the viability of Gram negative bacterial cells by increasing their antibacterial effects. [PERSON] et al. [115] reviewed the toxicity of nanomaterials on biomass and found that the chemical stability of nanoparticles of Ag, TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) have no adverse effects on microbes under anaerobic conditions, while Au nanoparticles showed low or no toxicity on microbes in anaerobic digestion, and CeO\({}_{2}\) nanoparticles presented the highest toxicity to both thermophilic and mesophilic microbes. As a result of metal ion release due to dissolution and corrosion of nanoparticles, the AD process was taxed. These toxic compounds can lead to obstruction of methane formation, a decrease in the methane content of biogas, or a complete failure of the process of methanogenesis.