The amino acid metabolism is particularly sensitive to environmental disturbances. Li et al. reported that environmental disturbances to microorganisms might result in increased levels of intracellular amino acids caused by the degradation of abnormal, misfolded proteins. Afterwards, the microorganisms would re-synthesize correctly folded proteins to maintain their normal functions in adverse circumstances. However, if adverse circumstances exerted a negative pressure so severe that the microbes could not withstand them, the cells would stop synthesizing proteins and the concentration of intracellular amino acids would decrease . Phosphoric acid on the other hand, is not only a vital component of the phospholipid bilayer, but also acts as an activator for a number of protein kinases that regulate important signal transduction pathways and make the cells induce stress responses against adverse environmental conditions . Therefore, the changes in the content of amino acids and free phosphoric acid shown in Fig 7 might be caused by stress mechanisms intrinsic to the microorganism. When exposed to low concentrations of MSNs, E. coli would resist adverse environmental conditions by producing more amino acids and accumulating free phosphoric acid, whereas high dosages of MSNs, which might exceed the tolerance range of E. coli, would result in degradation of amino acids and decrease of free phosphoric acid.