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).
