Adaptive Evolution of Lactobacillus casei under Acidic Conditions Enhances Multiple-stress Tolerance

Acid stress is an environmental condition commonly encountered by lactic acid bacteria in the gastrointestinal tract and fermented foods. In this study, adaptive evolution of Lactobacillus casei was conducted in MRS broth at 37 degrees C for 70 days, and three acid-tolerant strains were isolated. The evolved strains exhibited more than 60% increases in biomass than the parental strain when cultured at pH 4.3 for 54 h. Then, the tolerances of the evolved strains to acid stress, bile salt, and simulated gastrointestinal juice were determined. Acid tolerance was investigated by exposing the evolved strains and the parental strain in pH 3.3 and pH 2.5 of lactic acid and HC1-adjusted MRS broth respectively. The evolved strains showed higher survival rate than the parental strain. In addition, the evolved strains exhibited higher resistance to simulated gastric juice (pH 2.5), simulated intestinal juice (pH 8.0), and bile salt (1.6%). Analysis of the intracellular pH (pH) showed that the evolved strains exhibited higher pH, compared with that in parental strain. The enhanced tolerance of the evolved strains to various stresses observed in this study provides a possible procedure for the food industry to select environmental-resistant strains and may be important in the production of probiotics.