Boosting Probiotic Biomass of <i>Lactobacillus acidophilus</i> CCFM137 Through pH-Stat Morphological Control and Medium Optimization

The fermentation performance of <i>Lactobacillus acidophilus</i> is constrained by factors such as low cell density and fastidious nutritional and environmental requirements, which greatly limit its industrial-scale applications. This study aimed to develop an efficient fermentation condition for <i>L. acidophilus</i> CCFM137 through systematic optimization of both culture medium and environmental parameters, thereby enabling high-yield industrial-scale production of this strain. An optimized medium was developed, consisting of glucose (30 g/L), YEP FM503 (35 g/L), sodium acetate (5 g/L), ammonium citrate (2 g/L), K₂HPO₄ (2 g/L), MgSO₄·7H₂O (0.1 g/L), MnSO₄·H₂O (0.05 g/L), L-cysteine hydrochloride (0.5 g/L), and Tween 80 (1 mL/L), to achieve a viable cell count of 1.95 × 10⁹ CFU/mL, representing a 9.42-fold increase over that of standard MRS broth. Subsequent pH-stat fermentation trials in a 100 L fermenter using the optimized medium revealed morphological and growth characteristics of the strain in variable pH-stat environments. Optimal performance was observed under pH-stat 4.5 rather than the more commonly used 5.7, achieving maximum viable cell counts of 3.37 × 10⁹ CFU/mL, accompanied by a transformation of cell morphology toward shorter rod-shaped structures, as well as an increase in substrate utilization rate, cell recovery rate and lyophilization survival rate. The fermentation performance and cellular morphology of <i>L. acidophilus</i> CCFM137 were enhanced by both nutrient composition and pH environment. These results showed that this strategy has potential for application in high cell density fermentation of <i>L. acidophilus</i> CCFM137.