Antimicrobial properties of tannin extracts as animal feed

For many years, antibiotics were added to animal feed as additives to improve growth rates and reduce costs [1]. That caused, together with antibiotic over prescription, overuse and missuses in human medicine [2], an increase in antimicrobial resistance. The use of antimicrobial compounds as growth-promoting factors after 2006 has been forbidden by the European commission with the Regulation of the European Parliament and of the Council (EC) No 1831/2003 [3]. Without the use of antibiotics, the incidence of infectious diseases is increasing, and alternative means to control bacteria growth are necessary [1]. Tannins are natural compounds with proven antimicrobial properties and as such represent suitable alternatives [4]. The aim of our study was to determine the in vitro minimal inhibitory concentration (MIC) of tannins against Escherichia coli under different circumstances. We studied several pure compounds (vescalagin, castalagin, gallic acid and tannic acid) as well as various commercial plant extracts. We determined the minimal inhibitory concentrations of these compounds using two complementary methods (broth microdilution method by measuring optical density and by adding INT dye) that provided similar results. We also changed the composition of the media. Our results showed that minimal inhibitory concentration (MIC) highly depends on both the selected compound and media composition. Gallic acid has the highest MIC while tannic acid provides the best results (lowest MIC).

Fig. 1 Growth curves for the bacteria E. coli in media with increasing concentration of tannins. Elongation of lag phase can be observed at tannin concentration just below MIC (blue triangles).

• References

• 1 Redondo LM, Chacana PA, Dominguez JE, Fernandez Miyakawa ME. Perspectives in the use of tannins as alternative to antimicrobial growth promoter factors in poultry. Front Microbiol 2014; 5: 1-7
• 2 Michael CA, Dominey-Howes D, Labbate M. The Antimicrobial Resistance Crisis: Causes, Consequences, and Management. Front Public Health 2014; 2: 1-8
• 3 The European Parliament and the Council of the EU. . Regulation (EC) No 1831/2003 of the European Parliament and of the Council of 22 September 2003 on additives for use in animal nutrition. Official Journal. 2003 268. 29-43
• 4 Huang Q, Liu X, Zhao G, Hu T, Wang Y. Potential and challenges of tannins as an alternative to in-feed antibiotics for farm animal production. Anim Nutr 2018; 4: 137-150