Anaerobic transformation of quercetin-3-glucoside by bacteria from the human intestinal tract

Abstract

From human feces two phenotypically different types of bacteria were isolated on quercetin-3-glucoside as carbon and energy source. Isolates of one type were identified as strains of Enterococcus casseliflavus. They utilized the sugar moiety of the glycoside, but did not degrade the aglycon further. The sugar moiety (4 mM) was fermented to 5.5 ± 2.1 mM formate, 2.1 ± 0.7 mM acetate, 1.6 ± 0.3 mM l-lactate, and 1.3 ± 0.4 mM ethanol. The second type of isolate was identified as Eubacterium ramulus. This organism was capable of degrading the aromatic ring system. Growing cultures of Eubacterium ramulus converted 5 mM quercetin-3-glucoside to 1.7 ± 0.6 mM 3,4-dihydroxyphenylacetic acid, 7.6 ± 1.0 mM acetate, and 4.0 ± 0.4 mM butyrate. Molecular hydrogen, 3,4-dihydroxybenzaldehyde, and ethanol were detected in small amounts. Phloroglucinol was a transient intermediate in the breakdown of quercetin-3-glucoside. Eubacterium ramulus did not grow on the aglycon quercetin or the ring-fission intermediate phloroglucinol, but cleaved the flavonoid ring system when glucose was present as a cosubstrate. The most probable number of quercetin-3-glucoside-degrading bacteria determined in nine human fecal samples was 10⁷–10⁹/g dry mass. Isolates from these experiments were all identified as Eubacterium ramulus.