Potential and challenges of tannins as an alternative to in-feed antibiotics for farm animal production

Huang, Qianqian; Liu, Xiuli; Zhao, Guoqi; Hu, Tianming; Wang, Yuxi

The antimicrobial activities of tannins have long been recognized and the toxicity of tannins to bacteria, fungi and yeasts has been reviewed (Scalbert, 1991) . The mechanisms proposed so far to explain tannin antimicrobial activity include inhibition of extracellular microbial enzymes, deprivation of the substrates required for microbial growth, direct action on microbial metabolism through inhibition of oxidative phosphorylation, metal ions deprivation or formation of complexes with the cell membrane of bacteria causing morphological changes of the cell wall and increasing membrane permeability (Scalbert, 1991; Liu et al., 2013) . Evidences have shown that the microbial cell membrane is the primary site of inhibitory action by tannins (Mcallister et al., 2005; Liu et al., 2013) through cell aggregation and disruption of cell membranes and functions (Fig. 2) . Although protein precipitation is a universal property for all tannins, anti-microbial activity of tannins is microbial species-specific and is closely related to the chemical composition and structure of tannins. Generally, antimicrobial activity of tannins against Gram-positive bacteria has been reported to be greater than against Gram-negative bacteria (Ikigai et al., 1993; Smith and Mackie, 2004) , because Gramnegative bacteria possess an outer membrane that consists of a lipid bilayer structure which is composed of an outer layer of lipopolysaccharide and proteins and an inner layer composed of phospholipids. However, tannins especially CT isolated from several plants have been shown to possess strong activity against Gram-negative bacteria. It is worth noting that pathogenic bacteria such as Escherichia coli O157:H7, Salmonella, Shigella, Staphylococcus, Pseudomonas and Helicobacter pylori were all sensitive to tannins (Funatogawa et al., 2004; Doss et al., 2009; Banso and Adeyemo, 2010; Liu et al., 2013) . Wang et al. (2013) compared 12 tannins and found only CT isolated from purple prairie clover (Dalea purpurea Vent) and PT from brown alga (Ascophyllum nodosum) possessed strong anti-E. coli and anti-E. coli O157:H7 activity. Phlorotannins also have greater antimicrobial activity than CT and HT (Wang et al., 2009) . It has been shown that number of hydroxyl groups and liberation of hydrogen peroxide upon oxidation of tannins are 2 important factors responsible for the antimicrobial properties of tannins (Akagawa et al., 2003; Smith et al., 2003; Mueller-Harvey, 2006) . It has been proposed that flavonols with a trihydroxy B ring (gallocatechin) have a greater inhibitory effect on Streptococcus, Clostridium, Proteus and Staphylococcus species than catechin with a dihydroxy B ring (Sakanaka et al., 1989) . Similarly, the toxicities of epi-catechin gallate and epigallocatechin gallate towards Clostridium botulinum were greater than those of their ungallated counterparts-epicatechin and epigallocatechin (Okuda et al., 1985) . Because of the vast sources of tannins, which results in great diversity in their antimicrobial activities, screening and identification of tannins that are effective and specific to target microbes would continuously be a research endeavor.