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 (⌬⌿) 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 ⌬⌿ 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 ⌬⌿ may be a more important acid resistance strategy than maintaining a specific pH i value.

Roles of H2 uptake hydrogenases in Shigella flexneri acid tolerance

“…Enterobacteriaceae AR is highly complex and dependent on many factors (Foster, 2004;Richard & Foster, 2004). At least two AR stationary phase pathways have been described in S. flexneri.…”

“…Another acid-combating system involving Cl 2 influx (via chloride channels) has been proposed to prevent hyperpolarization due to excess positive charge stress in E. coli (Iyer et al, 2002). The influx of the chloride (in exchange for H + ) is thought to change the internal potential from positive to less positive (Richard & Foster, 2004). Conceivably, hya, due to a lower ability to generate cytoplasmic electrons, could be phenotypically rescued by addition of Cl 2 .…”

“…The AR2 pathway is the most efficient S. flexneri acidcombating pathway at pHs below 2.5 (Richard & Foster, 2004). It is dependent upon efficient glutamate uptake.…”

“…The genes involved in AR2 include decarboxylases (gadA and/or gadB) and an antiporter (gadC). Glutamate is taken into the cell via GadC, and decarboxylated via GadA/ B, which consumes an intracellular proton; the product, gamma aminobutyric acid (GABA), is exported out of the cell through the action of GadC (Richard & Foster, 2004). The net result is a neutralization of the cytoplasm, aiding acid tolerance.…”

The acid-resistance pathways of Shigella flexneri 2457T

“…This system appears to act by mopping up protons leaking into the bacterial cytosol through the decarboxylation of glutamate to gammaaminobutyric acid (GABA). GABA is then exchanged for external glutamate by the antiporter GadC, thereby maintaining the pH homeostasis of the cytoplasm, reversing the cell membrane potential to create an internal positive charge, and gradually alkalizing the extracellular medium (Castanie-Cornet & Foster, 2001;Richard & Foster, 2004). Thus, this pathway is dependent on glutamate being present in the acid-shock media (Lin et al, 1995).…”