L-Malic Acid Protects Lacticaseibacillus paracasei L9 from Glycodeoxycholic Acid Stress via the Malolactic Enzyme Pathway

Bile stress tolerance is a crucial characteristic of probiotics for surviving in the human gastrointestinal tract. The mechanism underlying the effect of l-malic acid on enhancing the glycodeoxycholic acid (GDCA) tolerance of Lacticaseibacillus paracasei L9 was investigated herein. Bile tolerance specificity assays revealed that Lc. paracasei L9 was more sensitive to GDCA than to taurocholic acid, glycocholic acid, and taurodeoxycholic acid. Notably, l-malic acid significantly enhanced the GDCA tolerance of Lc. paracasei L9 by increasing the pH of the medium. The role of the malolactic enzyme pathway in enhancing GDCA resistance was investigated using molecular techniques. Confocal laser scanning and scanning electron microscopy revealed that l-malic acid preserved membrane permeability and cellular morphology, thereby protecting bacterial cells from GDCA stress-induced damage. The study also demonstrated that l-malic acid enhanced bile tolerance in different species of lactobacilli. These findings provide a novel protective mechanism for coping with bile stress in lactobacilli.