Investigation of the mechanism of tannin antimicrobial action

Plant extracts have been shown to possess a range of biological activities, including antimicrobial effects [1], [2], [3]. Tannins influence bacterial growth through several mechanisms. One of these is the formation of coordination compounds with essential metal ions. The complex composition of the extracts makes it difficult to establish composition-to-activity and structure-to-activity relationships. The aim of our study was to isolate individual tannin components (vescalin, vescalagin, castalin, castalagin) from chestnut extracts and to analyse the influence of commercial extracts and pure tannins on the growth of Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Antibacterial activity was followed by monitoring the effect of tannins on the minimum inhibitory concentration (MIC), as well as on the duration of the lag phase. In the case of Escherichia coli, MIC values were found to increase proportionally to the concentration of the medium, while for tannin concentrations lower than the MIC, the duration of the lag phase increased exponentially with the increasing tannin concentration [4]. For Staphylococcus aureus dependencies were not so pronounced. Pure compounds with MIC values lower than those of the extracts best inhibit bacterial growth and thus contribute significantly to the antibacterial activity of tannin extracts. Therefore, the formation of coordination compounds between gallic acid, vescalin, castalin, vescalagin, and castalagin with Fe (II) ions was studied using UV-Vis spectroscopy and Jobʼs method ([Fig. 1]). It was determined that vescalin and castalin bind two iron (II) ions, while vescalagin and castalagin bind three iron (II) ions, which corresponds well to their MIC values [5].

Fig. 1 Left – MIC values of samples at differently concentrated media, right – Job plots of coordination compounds.

Publikationsverlauf

Artikel online veröffentlicht:
12. Dezember 2022

• References

• 1 Štumpf S, Hostnik G, Primožič M. et al. The Effect of Growth Medium Strength on Minimum Inhibitory Concentrations of Tannins and Tannin Extracts against E. coli. Molecules 2020; 25: 2947
• 2 Pizzi A. Tannins medical/pharmacological and related applications: A critical review. Sustainable Chemistry and Pharmacy 2021; 22: 100481
• 3 Tošović J, Bren U. Antioxidative Action of Ellagic Acid–A Kinetic DFT Study. Antioxidants 2020; 9: 587
• 4 Štumpf S, Hostnik G, Primožič M. et al. Generation Times of E. Coli Prolong with Increasing Tannin Concentration while the Lag Phase Extends Exponentially. Plants 2020; 9: 1680
• 5 Frešer F, Hostnik G, Tošović J, Bren U. Dependence of the Fe (II)-Gallic Acid Coordination Compound Formation Constant on the pH. Foods 2021; 10 (11) 2689