Antimicrobial
activity and modulation of gut microbiota

The importance
of dietary phenolics as antimicrobial compounds has increased in light of the
growing incidence of antimicrobial resistance by certain pathogens. The
antimicrobial activity of flavonoids and other polyphenols present in different
extracts of herbs and plants has been extensively documented (Smithand Mackie,
2004; Cushnie and Lamb, 2011) and reported in several recent reviews (Laparra
and Sanz, 2010; Requena et al.  2010;
Etxeberria et al. 2013). The microbiota present in the intestinal tract
could metabolize dietary polyphenols into more bioactive compounds with
different physiological significance and could also modify the composition
and/or activity of the intestinal bacteria population (Bustos et al.
2012). Thus, dietary phenolic compounds are often transformed by gut microbiota
and gut microbial population is modulated by dietary polyphenols in a two-way
phenolic–microbiota interaction. Polyphenols and their derivatives affect the
intestinal ecology as a significant proportion of them are not fully absorbed
but are metabolized in the liver, excreted through the bile as glucuronides and
accumulated in the ileal and colorectal lumen (Tzounis et al. 2008).
Substantial levels of unabsorbed dietary phenolic compounds exert significant
effects on the intestinal environment by suppressing or stimulating the growth
of some of the components of intestinal microbiota.

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           It has been shown in numerous in
vitro studies (Papadopoulou et al. 2005; Rodríguez-Vaquero et al.
2007; Ganan et al. 2009; Silván et al. 2013) that flavonoids
present in grape by-products have the capacity to inhibit the growth of certain
organisms, such as Staphylococcus aureus, Escherichia coli, Candida albicans
and Campylobacter. Until now, only a limited number of specific bacterial
species capable of dietary polyphenol degradation have been identified, for
example Eubacteriumramulus and Flavonifracter plautii, previously known as
Clostridium or biscindens (Braune et al. 2001; Schoefer et al.
2002). There have been very few studies on the interaction of polyphenol
compounds with intestinal microbiota in animal nutrition. The results obtained
by Viveros et al. (2011) on the use of GP and grape extract in birds’
diets also demonstrated and confirmed the antibacterial effect of polyphenols
found in these by-products with respect to certain intestinal bacteria. This
effect differs depending on the segment analyzed (ileum or caecum). The
inclusion of these by-products in birds’ diets exerted an antimicrobial effect
on Clostridium in the ileum, while in the caecum it was associated with an
increase in populations like Lactobacillus and Enterococcus.