Baseline ABTS Radical Scavenging Capacities of Common Microbial Media and the Impact of Sterilization Methods

Abstract

Background: Microbial culture media contain complex nutrient components such as yeast extract, peptides, and carbohydrates, which exhibit inherent redox properties. These baseline antioxidant activities can interfere with assays assessing the radical scavenging ability of microbial metabolites, particularly when media-derived effects are not properly controlled. Objectives: To quantitatively evaluate the intrinsic 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging capacities of five commonly used microbial media and to assess how media composition and sterilization methods contribute to assay interference. Methods: Five commercial liquid media — Tryptic Soy Broth (TSB), de Man-Rogosa-Sharpe (MRS) broth, Luria-Bertani (LB) broth (Miller), Nutrient Broth (NB), and Yeast extract-Malt extract (YM) broth — were tested for ABTS radical scavenging activity. Media were sterilized using either autoclaving (121°C, 20 min) or 0.22 μm syringe filtration. Radical scavenging (%) was calculated from absorbance at 734 nm after 15 min incubation with ABTS⁺ working solution. IC₅₀ values were determined by four-parameter logistic regression. Results: Baseline antioxidant capacity varied significantly depending on media composition, with the following potency order: MRS > TSB ≈ LB > YM > NB. Autoclaving further enhanced radical scavenging activity, particularly in carbohydrate-rich media such as MRS, likely due to the formation of melanoidin-like antioxidant compounds via heat-induced chemical reactions such as the Maillard reaction. Conclusions: Media-derived radical scavenging effects have substantial potential to confound antioxidant assays. Consideration of both media selection and sterilization method is essential to avoid misinterpretation of microbial functionality. This study provides practical guidance for minimizing assay interference and establishing reliable conditions in antioxidant-related microbiological research.