The study aimed at investigating how the antioxidant food components (polyphenols)
interact with the bacteria that represent the human intestinal microbiota. Seven
species of bacteria (isolated from the human intestine, and bought as pure bacterial
cultures in lyophilised form), used in the experiments, were Enterococcus caccae,
Escherichia coli, Lactobacillus sp., Bacteroides galacturonicus, Bifidobacterium
catenulatum, Ruminococcus gauvreauii and Eubacterium cylindroides. Plant material
with known health-promoting properties and a long tradition of use in folk medicine,
such as fresh fruits of raspberry, elderberry, cranberry, lingonberry, Japanese
quince and cornelian cherry, dried goji and Schisandra berries, red onion, bear’s
garlic, nettle, green tea and soybeans, as well as food supplements (commercial
pharmaceutical products: spirulina, noni juice and Citrosept) provided the source
of polyphenols. In addition, solutions of pure polyphenolic compounds (representing
various flavonoid classes and stilbenes) were used. The latter included (+)-catechin,
phloridzin, quercetin, rutin, kaempferol, naringin, naringenin, hesperidin, hesperetin
and resveratrol as polyphenols being the most frequently eaten with human diets.
At the first stage of the study, the material was examined for antioxidant activity,
total polyphenol content, and polyphenolic profile.
At the second stage, the effect exerted on individual bacteria species by pure
polyphenols, plant extracts and food supplements, used in different concentrations,
was evaluated. It was found that the natural antioxidant components and the
polyphenolic compounds present in plant material produce various effects on
intestinal bacteria, from stimulating, through neutral, up to bacteriostatic and
bactericidal, depending on the bacteria species. For extracts showing inhibitory
potential, the minimum inhibitory concentration (MIC) was determined. In search
of mechanisms behind the antioxidant and antimicrobial activity of polyphenols, the
role of the structural elements of their molecules was discussed.
The third stage of the research concerned the impact of intestinal bacteria on
the antioxidant potential and the concentration of selected antioxidant components
present in plant material. The changes that occurred due to the influence of
particular bacteria species were assessed. It was demonstrated that bacteria
representing the physiological intestinal microbiota of humans may biotransform
polyphenols through various pathways, producing derivatives of higher or lower
antioxidant potential (which has health implications). The increase in antioxidant
activity, however, may often be caused not by the metabolic changes of polyphenols,
but by their release from the bonds with proteins or other components present in
the reaction medium.
The results of the study provided a basis for formulating practical guidance
for the consumers of polyphenol-rich foods (among them dietary supplements)
and for the food industry.