Systems-level understanding of how Propionibacterium acidipropionici respond to propionic acid stress at the microenvironment levels: Mechanism and application

• Unraveling the acid-tolerant mechanism of Propionibacterium acidipropionici at microenvironmental levels.
• Low pHi promotes the P. acidipropionici to synthesize more H+-ATPase to pump the protons.
• The intracellular arginine, aspartic acid and glutamic acid concentrations increase in acid stress.
• Both ADI and GAD systems work for the survival of P. acidipropionici in the acidic environment.
• Addition of 20 mM arginine and aspartic acid improved PA production by 39.9%.

In previous work, three evolved Propionibacterium acidipropionici mutants with higher tolerant capacity of propionic acid (PA) were obtained by genome shuffling. Here, we attempted to unravel the acid-tolerant mechanism of P. acidipropionici by comparing the physiological changes between P. acidipropionici and three mutants. The parameters used for comparison included intracellular pH (pHi), NAD+/NADH ratio, H+-ATPase activity, and the intracellular amino acids concentrations. It was indicated that the acid tolerance of P. acidipropionici was systematically regulated. Specifically, low pHi promoted the P. acidipropionici to biosynthesize more H+-ATPase to pump the protons out of the cells, and as a result, the NAD+/NADH ratio increased due to the decreased protons concentration. The increased arginine, aspartic acid, and glutamic acid concentrations helped to resist the acidic environment by consuming more H+ and generating more ATP and NH3. Based on what was analyzed above, 20 mM arginine and aspartic acid were added during the shaker culture of P. acidipropionici, and the maximal PA titer reached 14.38 g/L, which was increased by 39.9% compared with the control.