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By Dr. Harmeet Kaur
The term plant tannins was first introduced in 1796 by Seguin to denote compounds present in plant extracts, which can combine with proteins of animal hides; prevent their putrefication and convert them into the leather. Tannins are water-soluble polyphenol derivatives naturally synthesized and accumulated by higher plants as secondary metabolites of molecular weights in the range of 500 and 3000 Da. In complexes with alkaloids, saccharides, and proteins, the molecular weight can go up to 30,000 Da.
The chemical structure of tannins mainly depends on the plant species from which they are obtained. They differ from another polyphenolic compound as their ability to precipitate proteins (astringent action). Presently, over 8000, different tannins from various plant parts have been isolated and chemically characterized. Due to medicinal properties, these are employed as hemostatic, antidiarrheal, and antibacterial compounds [1,2].
Based on standard structural features, the tannins can be classified into two main categories, hydrolysable tannins (HTs) and nonhydrolysable or condensed tannins (CTs). The HTs are generally partial or total esters of polyol moiety such as sugars with polyphenolic compounds like gallic acid (gallotannins; GTs) and ellagic acid, which is lactone form of hexahydroxydiphenic acid (elagllitannins; ETs). The GTs are mainly extracted from sumac (Rhus coriaria), tara (Caesalpinia spinosa), and gallnuts (Quercus infectoria and Rhus semialata).
Tannic acid is the simplest gallotannin which is formed by a pentagalloyl glucose molecule esterified by five gallic acid units. The ETs, are mainly present in chestnut wood (Castanea sativa), oak wood (Quercus robur, Quercus petraea and Quercus alba), and myrobalan (Terminalia chebula). Castalagin, castalin, casuarictin, grandinin, roburin A, tellimagrandin II, punicalagin and punicalin are some of the examples of ETs. HTs are generally hydrolyzed by weak acids or weak bases to produce carbohydrates and phenolic acids .
The CTs, also called procyanidins are polymers of flavonoids approximately 2 to 50 units joined by C-C bonds, which are not hydrolyzed by enzymes like tannases. These are further classified as proanthocyanidin and profisetinidin. Proanthocyanidins are naturally present in the skins and seeds of grapes (Vitis vinifera), and a common constituent of red wines. Profisetinidins are isolated from the mimosa (Acacia mollissima) and quebracho wood (Schinopsis lorentzii). CTs are much more resistant to microbial degradation in comparison to the HTs and exhibit stronger antiviral, antibacterial, and antifungal activity.
The chemical structure of plant tannins determines their biological activity. Moreover, tannins can act as potential: biological antioxidant, metal ion chelating agent, or depending on its concentration, as a complexing or precipitating agent (in low concentrations as a complexing, and in high as a precipitating agent). Recently the third class of tannins has been identified, the phlorotannins, present in many species of dark algae .
Several studies have been performed from time to time to prove the
antimicrobial potential of plant tannins. The corilagin and terchebulin,
ETs isolated from Terminalia chebula has demonstrated moderate
antimicrobial activity against multidrug-resistant Acinetobacter
baumannii with a minimum inhibitory concentration (MIC) of around 500
μg/mL. This study has also identified that the compound norwogonin from Scutellaria baicalensis was highly effective against A. baumanii .
In a similar study, the 50% aqueous ethanolic extracts of Acalypha wilkesiana and Acalypha hispida leaves having constituents of gallic acid, corilagin and geraniin have shown good antimicrobial effect against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis and Candida pseudotropicalis .
In another study, the corilagin has demonstrated antimicrobial activity against E. coli, S. aureus and Candida albicans and found that inhibition of E. coli and C. albicans by corilagin is due to disruption of cell membrane and enhanced permeability. Further the activity against S. aureus is due to action on the proteins Fib, Sae R and Sar S. Protein Tye 7p was revealed to be the possible target in C. albicans .
Punicalagin is an ET, found in α and β forms in pomegranates (Punica granatum). Punicalagin, tannic acid, castalagin and geraniin have shown moderate activity against genus Vibrio and S. aureus and weak activity against E. coli. It has been noticed that the punicalagin from Punica granatum fruits are more potent against methicillin-resistant S. aureus (MRSA) than the punicalagin obtained from Terminalia citrina .
Tannic acid (an gallotannin) has shown significant antibacterial action against S. aureus. Tannic acid is also known as a safe food additive present in many foods like grapes, cocoa beans, and tea and has shown its inhibitory effect on the growth of the intestinal bacteria like Clostridium perfringens, Bacteroides fragilis, E. coli and Enterobacter cloacae. Mechanistically the tannic acid had greater binding efficiency with iron, and thus, it may act as a siderophore to chelate iron from medium and make iron unavailable to the microorganisms .
Funatogawa et al. evaluated nine HTs i.e. tellimagrandin I (TG-I), tellimagrandin II (TG-II), acid-treated TG-I, acid-treated TG-II, agrimoniin, geraniin, oenothein A, epigallocatechin gallate (EGCG) and epicatechin (EC) for the bacterial membrane-disrupting activity by labelling with carboxyfluorescein. All the plant tannins have shown considerable membrane-damaging activity with the highest activity of EGCG and minimal for EC. The same nine compounds have also demonstrated good antibacterial activity towards Helicobacter pylori but no activity against E. coli, which naturally found in the gastrointestinal tract of humans .
The plant tannin profile of Cytinus hypocistis and C. ruber shown a significant amount of GTs, in particular 1-O-galloyl-β-D-glucose which have demonstrated activity against three Gram-positive bacterial species (S. aureus, S. epidermidis, Enterococcus faecium) but no effect on Gram-negative strains (P. aeruginosa and K. pneumonia). Interestingly, the suppressive activity of ethanolic extracts of C. hypocistis and C. ruber on biofilm formation of S. epidermidis has been observed .
The CTs from the stem barks of the Erythrophleum guineensis; demonstrated antimicrobial activity towards seven strains of microorganisms including three yeasts and four bacteria which known to be involved in food poisoning and acute infectious diarrhea. Among these, highest activity has been found against S. aureus, C. krusei and C. parapsilosis .
In another study, corilagin has shown a significant synergism with the antibiotics like cefmetazole, oxacillin, imipenem and benzylpenicillin against MRSA and also found to be specific with β lactams by 100- to 2000-fold but little or no synergism with, other antibiotics like ofloxacin, streptomycin, tetracycline and vancomycin .
Shiota et al. have reported that corilagin and tellimagrandin I isolated from Arctostaphylos uva-ursi and Rosa canina remarkably reduced the MIC of β-lactams in MRSA. The mechanistic studies indicated that inactivation of penicillin-binding proteins (PBPs), especially of PBP2 (PBP2a), by corilagin or tellimagrandin I was the major reason for the remarkable reduction in the resistance level of β-lactams in MRSA. Furthermore, corilagin and tellimagrandin I also suppressed the β-lactamase activity to a small extent .
The peel extract of the plant P. granatum has shown higher antifungal activity against Aspergillus niger . The ethanolic extract of peel and pericarp extract also contain tannins punicalagin and galladydilacton and inhibiting effect on the growth of fungi. The change in the morphology of hyphae and cell membrane in C. albicans and C. krusei, have proven potential of the tannins against Candida genus .
Punicalagin is also found in the aqueous extract of the peels of P. granatum and the extract shows considerable effects against Stemphylium botryosum, Alternaria alternata, and Fusarium species. Methanolic extract of pomegranate flower has also shown a good results against C. albicans.
In one study, GTs from unripe mango has expressed potent activity against anthracnose-resistant strains of Colletotrichum gloesoporioides and Alternaria alternate than the susceptible or non-resistant strains and the activity diminished on ripening of the fruit .
The ETs obtained from Euphorbia antisyphilitica has antifungal activity against four fungi viz Fusarium oxyzporum, A. alternata, Colletotrichum gloeosporoides and Rhizoctnia . The phenolic extract of C. ladanifer is rich in punicalagin gallate, and has shown activity against C. albicans, C. glabrata and Candida parapsilosis .
In another study, Morey et al. evaluated the effect of CT-rich fraction of Stryphnodendron adstringens on in vitro and in vivo growth of C. tropicalis, and on yeast adhesion properties. The plant tannin-rich fraction exhibited a fungistatic effect with MIC concentration ranging from 0.5 to 8.0 μg/mL with a significant reduction in biofilm mass and reduction in yeast adherence on HEp-2 cells .
The mechanisms of plant tannin toxicity are generally related to some of their characteristic physicochemical properties, particularly astringency. The addition of a ligand which can compete with microbial ligands such as enzymes can reduce microbial inhibition by tannins. The astringent character of plant tannins may induce complexation with enzymes or substrates. They can directly affect the metabolism by inhibiting oxidative phosphorylation by mitochondria and inhibition of the electron transport chain. They can affect the integrity of cell membranes. .
Another toxicity mechanism involves the complexation of metal ions by plant tannins. Biological systems, including microorganisms, are highly dependent on the metal ion status of the environment. Antimicrobial activity through iron depletion is particularly well documented. For example, infection of humans by E. coli is subsided by the iron-chelating lactoferrin present in human milk but restored by an excess of iron. Tannins might exert their antimicrobial action through a similar mechanism. Most of the plant tannins have more than two o-diphenol groups in their molecule, which can form chelates with many metal ions such as ferric or cupric ions also .
The oral route is unfavorable for HTs as they are hydrolysed in the intestinal pH and have poor pharmacokinetic parameters. Urolithins-the metabolites of ETs-have to be studied for its pharmacological and toxicological effects in detail to ensure the health benefits. Moreover, HTs have higher idiosyncrasy, that is, has variable pharmacokinetic property among the individuals studied. Hence, a suitable oral drug delivery system has to be formulated for HTs to achieve bioavailability. However, the intraperitoneal route, injection, of administration of HTs has proven to be quite efficient because the plasma concentration of ellagic acid (hydrolysed product of ETs) has increased, and the elimination was protracted .
Gallotannins (GTs) and Ellagitannins( ETs) are the categories of hydrolysable tannins(HTs). They contain ester bonds and can be hydrolysed, or destroyed, by the acidic pH of the stomach. So they are not effective when taken by mouth.
Condensed plant tannins(CTs) contain flavanoid units joined by carbon-carbon bonds which are much stronger and resistant to hydrolysis and can be effective orally.
Plant Tannins Conclussion,
Plant tannins are astringent, bitter-tasting plant polyphenols having sufficient hydroxy and carboxyl groups to form stable complexes with proteins and other macromolecules. Over the last century, studies on plant tannins have moved from organoleptic properties in foods and industrial applications (leather production) to the present-day research into biological properties allied with disease prevention.
These secondary plant metabolites have demonstrated their antimicrobial potential when used alone or in combination with the existing antibiotics. They can interact with bacterial cell walls, metabolic enzymes and essential elements required for survival of microorganisms. Hence, more emphasis should be given to this field since these phytochemicals can act through different mechanisms in comparison to the conventional antibiotics and thus can have the potential in reversing microbial resistance or acting through a pathway different from those of conventional antibiotics.
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Dr. Kaur's References
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