Study of the inhibitory activity of phenolic compounds found in olive products and their degradation by Lactobacillus plantarum strains

Lactobacillus plantarum is the main species responsible for the spontaneous fermentation of Spanish-style green olives. Olives and virgin oil provide a rich source of phenolic compounds. This study was designed to evaluate inhibitory growth activities of nine olive phenolic compounds against four L. plantarum strains isolated from different sources, and to explore the L. plantarum metabolic activities against these phenolic compounds. None of the nine compounds assayed (oleuropein, hydroxytyrosol, tyrosol, as well as vanillic, p-hydroxybenzoic, sinapic, syringic, protocatechuic and cinnamic acids) inhibited L. plantarum growth at the concentration found in olive products. Oleuropein and tyrosol concentrations higher than 100 mM were needed to inhibit L. plantarum growth. On the other hand, sinapic and syringic acid showed the highest inhibitory activity since concentrations ranging from 12.5 to 50 mM inhibited L. plantarum growth in all the strains analyzed. Among the nine compounds assayed, only oleuropein and protocatechuic acid were metabolized by L. plantarum strains grown in the presence of these compounds. Oleuropein was metabolized mainly to hydroxytyrosol, while protocatechuic acid was decarboxylated to catechol. Metabolism of oleuropein was carried out by inducible enzymes since a cell-free extract from a culture grown in the absence of oleuropein was unable to metabolize it. Independent of their isolation source, the four L. plantarum strains analysed showed similar behaviour in relation to the inhibitory activity of phenolic compounds, as well as their ability to metabolize these compounds.