Role of E. coli acid resistance systems in proton motive force formation during fermentation

The regulation of intracellular pH (pH_in) and the maintenance of a proton motive force (Δp) are vital processes for cellular viability. Escherichia coli employs different amino acid decarboxylase acid resistance (AR) systems: specifically, Gad (glutamate-dependent), Cad (lysine-dependent), and Adi (arginine-dependent) systems for pH_in and pH_ex regulation under acidic conditions. This study elucidated the pH-dependent roles of these AR systems in the formation and regulation of the Δp components (membrane potential [ΔΨ] and transmembrane proton gradient [ΔpH]) and proton flux rate (J_H⁺) during fermentation at different levels of acid stress: (no stress [pH 7.6], mild acid stress [pH 6.5], moderate acid stress [pH 5.8], and sublethal acid stress [pH 5.4]). The regulatory role of AR systems in Δp and pH homeostasis demonstrated significant special contingent at levels of acid stress. Moreover, AR systems affected the Δp components and the proton flux rate from the cytoplasm to the external environment. Under no acid stress, the Gad system played a role in the regulation of ΔpH by maintaining a higher extracellular pH (pH_ex). The Adi system demonstrated significance in balancing of Δp components, and the observed compensatory variation in balancing both ΔΨ and ΔpH in ΔadiY mutant emphasized the critical role of Adi in maintaining Δp stability under mild acid stress, pH 6.5. The Cad system was important in regulating ΔpH under moderate acid stress levels by influencing pH_in. Under sublethal acid stress, all AR systems were crucially involved in ΔpH regulation and J_H⁺. Thus, our data show that each AR system possesses a crucial and distinct role in Δp formation that is highly dependent on the acid stress level. Moreover, the role of one AR system was, in most cases, not fully compensated by the others, underscoring the importance of these AR systems individually in pH homeostasis in E. coli during fermentation.