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Applied and Environmental Microbiology, June 2003, p. 3406-3411, Vol. 69, No. 6
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.6.3406-3411.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Increasing the Oxidative Stress Response Allows Escherichia coli To Overcome Inhibitory Effects of Condensed Tannins

Alexandra H. Smith,^1,2, James A. Imlay,³ and Roderick I. Mackie^1,4*

Department of Animal Sciences,¹ Division of Nutritional Sciences,⁴ Department of Microbiology, University of Illinois, Urbana, Illinois 61801,³ Animal Nutrition and Animal Products Institute, Irene 0062, South Africa²

Received 29 July 2002/ Accepted 25 February 2003

Tannins are plant-derived polyphenols with antimicrobial effects. The mechanism of tannin toxicity towards Escherichia coli was determined by using an extract from Acacia mearnsii (Black wattle) as a source of condensed tannins (proanthocyanidins). E. coli growth was inhibited by tannins only when tannins were exposed to oxygen. Tannins auto-oxidize, and substantial hydrogen peroxide was generated when they were added to aerobic media. The addition of exogenous catalase permitted growth in tannin medium. E. coli mutants that lacked HPI, the major catalase, were especially sensitive to tannins, while oxyR mutants that constitutively overexpress antioxidant enzymes were resistant. A tannin-resistant mutant was isolated in which a promoter-region point mutation increased the level of HPI by 10-fold. Our results indicate that wattle condensed tannins are toxic to E. coli in aerobic medium primarily because they generate H₂O₂. The oxidative stress response helps E. coli strains to overcome their inhibitory effect.

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* Corresponding author. Mailing address: 132 Animal Sciences Laboratory, Department of Animal Sciences, 1207 W. Gregory Dr., Urbana, IL 61801. Phone: (217) 244-2526. Fax: (217) 333-8804. E-mail: r-mackie{at}uiuc.edu.

Present address: Center for Biomedical Inventions, Southwestern Medical Center, University of Texas, Dallas, TX 75229.

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Applied and Environmental Microbiology, June 2003, p. 3406-3411, Vol. 69, No. 6
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.6.3406-3411.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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