Screening, identification, and mechanism elucidation of zearalenone-reducing lactic acid bacteria from Chinese sourdough

Abstract:

[Background] Zearalenone (ZEN), a mycotoxin with estrogenic effects produced by Fusarium, is one of the pollutants that seriously jeopardize human health and agricultural safety worldwide. As a class of food-grade non-pathogenic microorganisms generally recognized as safe (GRAS), lactic acid bacteria have been proved to have good mycotoxin-reducing ability in recent years, demonstrating the application potential in safeguarding food safety. [Objective] To screen the lactic acid bacteria capable of reducing ZEN from seven Chinese sourdough samples collected from Shandong, Henan, and Gansu, and investigate the reducing mechanisms of ZEN by the bacteria. [Methods] The strains were isolated by the dilution-plate coating method. Ultra-high performance liquid chromatography was employed to screen the strains with ZEN-reducing activity, mass spectrometry (Q exactive) to identify the metabolites, and transmission electron microscopy (SEM) to observe the morphology of the strains. The adsorption effects were examined at different initial toxin concentrations and bacterial concentrations, and a kinetic model was fitted. Fourier transform infrared spectroscopy (FTIR) was employed to identify the functional groups involved in the adsorption of ZEN and elucidate the adsorption mechanism of ZEN. [Results] A total of 63 strains of Lactobacillus were isolated from the preliminary screening, and then three strains with ZEN-reducing activity were obtained. Strains 6-8 and 18-2 were identified as Levilactobacillus brevis, and strain 12-6 as Acetobacter tropicalis. The two strains of L. brevis had ZEN-degrading effects, with the degradation rates of 85.5% and 87.3%, respectively, within 48 h at ZEN concentration of 10 mg/L. The mass spectrometry results showed that the degradation product was α-ZEL. The other strain had a ZEN-adsorbing effect, with the adsorption rate reaching 62.9% within 20 min at a bacterial concentration of 4.26×10¹⁰ CFU/mL and a ZEN concentration of 10 mg/L. Moreover, the adsorption rate increased by 20% after strain inactivation. The adsorption process was fitted with both pseudo-first-order and pseudo-second-order models. Fourier transform infrared spectroscopy (FTIR) showed that the main adsorption sites of strain 12-6 were peptidoglycan and teichoic acid in the cell wall, involving hydroxyl, methenyl, carboxyl, and amide groups. [Conclusion] L. brevis 6-8 and 18-2 showed strong degradation capacity against ZEN, and A. tropicalis12-6 had a strong adsorption capacity for ZEN. The adsorption kinetics conformed to pseudo-first-order and pseudo-second-order models, with the adsorption sites at teichoic acid and peptidoglycan in the cell wall. This study provides a theoretical basis for the application of lactic acid bacteria in the removal of harmful substances from food and feed.