The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle, is a series of enzyme-catalyzed chemical reactions which occur in many bacteria. The contents of tricarboxylic acid cycle-related metabolites (e.g. lactate, 4-hydroxybutyrate, succinate, glycine) showed a similar change to the changes of amino acids. This result suggested that the metabolic fluxes in the TCA cycle were increased when E. coli was exposed to low concentrations of MSNs, but high dosages of MSNs suppressed the TCA cycle. The levels of metabolites related to the TCA cycle are also decreased under different stress conditions in E. coli .