Whole-genome sequencing of Lactobacillus helveticus HP-B1137 and its biotransformation capacity toward calycosin-7-O-β-D-glucoside.

Abstract

Introduction: Astragalus, as a traditional tonic herb, has alycosin-7-O-β-D-glucoside as one of its main active components.

Methods: In this study, a strain of Lactobacillus helveticus HP-B1137 was isolated from raw milk in Mudan District, Heze City, Shandong Province. The probiotic properties, and ability to biotransform calycosin-7-O-β-D-glucoside were systematically evaluated.

Results and discussion: Genome analysis revealed that HP-B1137 possesses a circular chromosome (2,098,901 bp) comprising 2,161 CDs, 63 tRNAs and 12 rRNAs. Two circular plasmids were also identified, with sizes of 23,906 bp and 6,414 bp and GC contents of 38.54 and 34.49%, respectively. Functional annotation showed that the strain is rich in carbohydrate metabolism genes, with 29 glycoside hydrolase genes annotated by the CAZy database, providing a genetic basis for its glycoside-hydrolyzing capability. In vitro probiotic tests demonstrated that the strain maintained high viable cell counts after 3 min of heat treatment at 50 °C, achieved a survival rate of 108.33% after 150 min in artificial gastric juice at pH 2.7. It also inhibited 11 pathogenic bacteria, producing inhibition zones ranging from 9.83 to 16.37 mm. Whole-cell transformation experiments indicated that HP-B1137 could nearly completely convert 0.5 mg/mL calycosin-7-O-β-D-glucoside into calycosin within 36 h. Further cloning and expression of the recombinant BgsH enzyme revealed optimal reaction conditions at 37 °C and pH 7.0; under optimized conditions-0.1% substrate concentration, 10 U/mL enzyme activity, and 12-h incubation-the conversion efficiency exceeded 96.3%. In summary, L. helveticus HP-B1137 is a functional strain integrating efficient biotransformation, excellent probiotic characteristics, tolerance to extreme environments, and broad-spectrum antimicrobial activity, showing significant potential for developing functional foods derived from Astragalus and green bio-manufacturing of active aglycones.