Urolithin A Production by Rare Lactic Acid Bacteria Isolated from Traditional Chinese Qu Starter Cultures

Objectives: To identify food-origin lactic acid bacteria (LAB) capable of converting ellagic acid (EA) to urolithin A (UA) under chemically defined conditions, characterise their biotransformation kinetics, and evaluate preliminary probiotic safety criteria. Methods: Fifty-two LAB isolates from traditional Chinese Qu starter cultures were screened for EA-to-UA conversion in MRS broth with 20 µM pure EA (37°C, 72 h, microaerophilic). UA and EA were quantified by HPLC; UA identity was confirmed by HR-ESI-MS. Producers were identified by 16S rRNA sequencing and subjected to time-course fermentation (0–98 h). Safety assessment covered acid and bile salt tolerance, hemolytic activity, and antibiotic susceptibility. Findings: Screening identified two UA-producing strains from 52 isolates (3.8%), both from Qu starters: Lactiplantibacillus plantarum TCQ1 (99.7% 16S rRNA identity to type strain ATCC 14917ᵀ) and Limosilactobacillus fermentum TCQ3 (99.9%). Under defined conditions, TCQ1 produced 7.93 ± 0.14 µM UA (39.65 ± 0.69% molar conversion), and TCQ3 produced 6.21 ± 0.31 µM UA (31.06 ± 1.57%; t = 8.68, p = 0.001). Time-course analysis showed strain-specific kinetics: TCQ1 peaked at 6.482 ± 0.918 µM at 56 h (44.9% yield) and TCQ3 at 5.575 ± 0.291 µM at 72 h (36.1%); both depleted EA to ≈100% by 98 h with UA remaining stable thereafter (all post-peak p > 0.05). Both strains were γ-hemolytic, retained ~103 CFU/mL after 3 h at pH 2.0, and maintained >94% survival at 0–0.30% (w/v) bile, satisfying FAO/WHO preselection criteria. Novelty: TCQ1 and TCQ3 are the first UA-producing LAB from Chinese Qu starter cultures and represent the first characterisation of EA-to-UA conversion by food-environment LAB isolates using pure EA under defined conditions. TCQ3 is only the second L. fermentum strain with confirmed UA-producing capacity. Both strains serve as research platforms for enzymatic pathway investigation and UA-enriched fermented food development. Keywords: Urolithin A, Lactic Acid Bacteria, Ellagic Acid, Biotransformation, Qu Starter Cultures, Probiotic Screening