Bacterial Degradation of Bacteriostatic Polyphenols, Tannins

Cherian Gloria Susan,Raja Madhan

doi:10.30799/jnpr.100.21070201

Abstract

Tannin degradation by bacteria has not been studied much as tannins are commonly known to be bacteriostatic due to enzyme inhibition, substrate deprivation, and the enzyme activity on the bacterial cell wall. However, about a handful of bacteria have been found to tolerate certain concentrations of tannin. This study focuses on isolating and identifying bacteria from decaying portions of tree bark for tannase production and effective catalysis of ester bond hydrolysis in tannins. Different concentrations of commercial tannic acid were used as the sole carbon source on mineral salt medium (MSM) agar plates, to test the maximum tolerable concentrations (MTCs) by the isolates. Tannin degradation was confirmed by a visual reading method and bacterial tannase activity and the biodegradation percentage were determined. One particular isolate was identified to have 50 g/L MTC of tannin, with a tannase activity of 51.61 U/mL that is optimum after 96 hours of incubation. The 16s rRNA sequencing results showed that the isolate belonged to Bacillus genus and the resulting bacterial strain isolate was found to be a new strain of Bacillus subtilis which was submitted to GenBank under the accession number MH330408.

Highlights

Tannins are complex recalcitrant compounds that are consumed by humans on a regular basis as they are universally present in plants and represent the fourth most abundant group of secondary metabolites after cellulose, hemicellulose and lignins [1]
Crude enzyme containing inoculum that had been incubated for up to 72 hours was directly poured into wells created on purely agar containing plates, with tannic acid added as the substrate for enzyme activity
Addition of tannic acid to nutrient agar forms a tannin protein complex; cleavage of this complex by bacteria producing tannase forms a dark zone of clearance

Summary

Introduction

Tannins are complex recalcitrant compounds that are consumed by humans on a regular basis as they are universally present in plants and represent the fourth most abundant group of secondary metabolites after cellulose, hemicellulose and lignins [1] These are naturally occurring water soluble polyphenols that are found in many parts of plants such as leaves [2], fruits [3], food grains [4], bark and wood etc., [5]. Tannins in other words are water-soluble polyphenols with a molecular mass ranging from 0.3 – 5 kD that are known to be inhibitory against several organisms [7] and are recalcitrant to biodegradation [8] Their role primarily, has been studied to guard the vulnerable parts of plants from microbial attack by inactivating the invasive microbial extracellular enzymes [9]. Foods rich in tannins are of low nutritional value as they form irreversible complexes with proteins, starch and digestive enzymes [12]

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