A Modified and Improved Assay Based on Microbial Test System (MTS) to Evaluate Antioxidant Activity

An assay based on microbial test system (Escherichia coli) was developed to evaluate the antioxidant capacity of phytochemicals, extracts of plants, fruits, and vegetables. E. coli was cultured in Luria-Bertani (LB) medium. Quercetin was used as a standard in microbial test system (MTS) assay because it had high antioxidant activity and was easily and economically obtained. The cultivation times of E. coli entrance into mid-log phase, before and after H2O2 addition for MTS assay were investigated, respectively. The concentrations of H2O2 and tested compounds for MTS assay were also studied. The value (mu(30)), t = 30 min, the specific growth rate of E. coli in medium containing samples and 6.0 mM H2O2/the specific growth rate in medium containing only H2O2 was calculated to evaluate the antioxidant activity. Pure compounds (quercetin, quercitrin, hyperoside, rutin, and kaempferol-3-O-alpha-L-rhamnoside), fruits (apple, banana, kiwi fruit), and vegetable (tomato) were used to verify this assay, and the results were compared to three in vitro antioxidant assays (2,2A '-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS), 2,2-diphenyl- 1-picrylhydrazyl (DPPH), and ferric reducing antioxidant power (FRAP) assay). The results indicated the MTS assay based on E. coli without transferring the antioxidant genes under peroxide stress would be useful to define the action of extracts or pure compounds as antioxidants. The proposed method is useful to evaluate antioxidant activity and more biologically relevant because it accounts for some aspects of uptake and location of antioxidants within bacterial cells.