Escherichia coli Glutamate- and Arginine-Dependent Acid Resistance Systems Increase Internal pH and Reverse Transmembrane Potential

Abstract

Due to the acidic nature of the stomach, enteric organisms must withstand extreme acid stress for colonization and pathogenesis. Escherichia coli contains several acid resistance systems that protect cells to pH 2. One acid resistance system, acid resistance system 2 (AR2), requires extracellular glutamate, while another (AR3) requires extracellular arginine. Little is known about how these systems protect cells from acid stress. AR2 and AR3 are thought to consume intracellular protons through amino acid decarboxylation. Antiport mechanisms then exchange decarboxylation products for new amino acid substrates. This form of proton consumption could maintain an internal pH (pH i ) conducive to cell survival. The model was tested by estimating the pH i and transmembrane potential (ΔΨ) of cells acid stressed at pH 2.5. During acid challenge, glutamate- and arginine-dependent systems elevated pH i from 3.6 to 4.2 and 4.7, respectively. However, when pH i was manipulated to 4.0 in the presence or absence of glutamate, only cultures challenged in the presence of glutamate survived, indicating that a physiological parameter aside from pH i was also important. Measurements of ΔΨ indicated that amino acid-dependent acid resistance systems help convert membrane potential from an inside negative to inside positive charge, an established acidophile strategy used to survive extreme acidic environments. Thus, reversing ΔΨ may be a more important acid resistance strategy than maintaining a specific pH i value.