Molecular mechanisms for exopolysaccharides synthesis in Lactobacillus helveticus: Relationship between structural characteristics and genomics

Exopolysaccharides (EPS) play an important role in the processing of fermented dairy products, yet the regulation of their biosynthesis and structure remains largely unknown. In this study, we determined the genomes of 46 strains of Lactobacillus helveticus along with the production and molecular weight of their EPS, and for the first time, employed genome-wide association studies to explore the molecular mechanisms influencing EPS biosynthesis. The main findings reveal that hypothetical protein and UDP-N-acetyllucosamine 2-heptanedioic acid were significantly associated with both EPS production and number average molecular weight (Mn), but the trend is opposite. Alpha-D-GlcNAc alpha-1,2-L-rhamnosyltransferase and UDP-galactopyranose mutase were only considerably associated with EPS Mn, but not with EPS production. The C → T mutation in the exopolysaccharide biosynthesis protein resulted in a significant decrease in EPS Mn, whereas the same mutation in the Wzz/Phepe/Etk N-terminal domain containing protein significantly elevated EPS production. Additionally, it was found that high Mn EPS can enhance the texture, water-holding capacity, and rheological properties of fermented milk. This study demonstrates that the combination of comparative genomics and genome-wide association studies can effectively analyze the structural differences of EPS in lactic acid bacteria, and confirms that high Mn EPS significantly improves the gel properties of fermented milk.