Recent advances of pH homeostasis mechanisms in Corynebacterium glutamicum

Corynebacterium glutamicum is generally regarded as a safe microorganism, and widely used in the large-scale production of various amino acids and organic acids, such as L-glutamate, L-lysine and succinic acid. During the process of industrial fermentation, C. glutamicum is usually exposed to varying environmental stresses, such as variations in pH, salinity, temperature, and osmolality. Among them, pH fluctuations are regarded as one of the most frequent environmental stresses in microbial fermentation. In this review, we summarize the current knowledge of pH homeostasis mechanisms adopted by C. glutamicum for coping with low acidic pH and high alkaline pH stresses. Facing with low pH environments, C. glutamicum develops a variety of strategies to maintain intracellular pH homeostasis, such as lowering intracellular reactive oxygen species, the improvement of potassium transport, the regulation of mycothiol-related pathways, as well as the repression of sulfur assimilation. While during alkaline pH stresses, the Mrp-type Na⁺/H⁺ antiporters are shown to play a dominant role in conferring C. glutamicum cells resistance to alkaline pH. Furthermore, we also discuss the general strategies and prospects on metabolic engineering of C. glutamicum to improve alkaline or acid resistance.