Strain-specific nitrogen source preferences and pH-controlled fermentation of Lactobacillus gasseri, L. acidophilus, and L. helveticus for industrial probiotic production.

This study aimed to identify growth-limiting factors and optimize fermentation conditions for three probiotic lactobacilli (Lactobacillus gasseri, L. acidophilus, and L. helveticus) that show suboptimal performance in industrial processes, with the goal of enhancing cellular proliferation and production efficiency. Four commercial nitrogen sources were evaluated for their impact on viable cell counts, pH, and morphology. Scale-up fermentation was subsequently conducted in a 100-L pH-stat fermenter to examine pH-dependent growth kinetics. Results revealed strain-specific nitrogen preferences: L. helveticus exhibited robust growth across all yeast extracts, whereas L. acidophilus and L. gasseri displayed significantly higher viable cell counts (P < 0.05) in yeast extract 503 (rich in nucleotides) compared to other nitrogen sources, accompanied by reduced cell lengths. Furthermore, pH-stat fermentation at 4.5 promoted growth (2.96-3.47 × 10⁹ CFU/mL) and shortened cell morphology for all three strains. These findings reveal how nucleotides in nitrogen sources and acidic pH regulate growth of lactic acid bacteria via nitrogen-pH-morphology interplay, offering important implications for industrial-scale production optimization.