The Effect of Growth Medium Strength on Minimum Inhibitory Concentrations of Tannins and Tannin Extracts against E. coli.

Sara Štumpf,Urban Bren,Gregor Hostnik,Mateja Primožič,Maja Leitgeb,Juha-Pekka Salminen

doi:10.3390/molecules25122947

Abstract

In this study the effect of growth medium strength on the minimum inhibitory concentration (MIC) of different tannins and tannin extracts against Escherichia coli was systematically investigated for the first time. Three pure compounds (vescalagin, castalagin and gallic acid) and five extracts (chestnut, quebracho, mimosa, Colistizer and tannic acid) were studied. Broth microdilution was assayed and bacteria were grown using different growth medium strengths varying from half to double the concentration recommended by the producer. MICs were determined using the iodonitrotetrazolium chloride (INT) dye or turbidity measurements. It was observed that MIC values depend on the growth medium strength. With an increase in the growth medium concentration MIC values rose roughly linearly for all samples, while their relative order remained unchanged, indicating that a direct interaction of tannins with growth medium nutrients represents the likely source of their antimicrobial activity. Understanding the effect of growth medium strength can finally yield a plausible explanation for the observed variation in MIC values reported in the scientific literature as well as provide help in planning proper applications of tannins in the livestock production.

Highlights

The discovery of antibiotics represents one of the greatest breakthroughs of modern medicine.They are often used for treating diseases of domestic animals
We showed that dependence of minimum inhibitory concentration (MIC) values on media strength is valid for hydrolysable tannins and pure compounds by determining MIC values of crude commercial extracts
MIC values of several tannin extracts and pure compounds were determined against E. coli using either measurement of optical density (OD) or the addition of iodonitrotetrazolium chloride (INT) dye and both methods provided reproducible and similar results

Summary

Introduction

The discovery of antibiotics represents one of the greatest breakthroughs of modern medicine They are often used for treating diseases of domestic animals. The use of antibiotics for non-therapeutic purposes as growth-promoting factors in the European Union has been banned after 2006 [1], while it remains allowed in several parts of the world. This widespread application of antibiotics led to an increase in antimicrobial resistance in animal microbiota and later generated a transfer of resistance genes from animal to human bacteria strains, which together with misprescription and overprescription of antibiotics to patients caused the development of multiple drug-resistant pathogenic bacteria [2,3,4,5,6,7]. Without the application of antibiotics, the incidence of Molecules 2020, 25, 2947; doi:10.3390/molecules25122947 www.mdpi.com/journal/molecules

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