A Study of the Responses of SDH, ICD, MDH and G6PD to acid Shock Generated Acid Tolerance in Escherichia Coli

Abstract: Escherichia coli have an optimum pH range of 6 to7 for growth and survival and are hence, called neutrophiles. When challenged by low pH, protons enter the cytoplasm; as a result mechanisms are required to alleviate the effects of lowered cytoplasmic pH. Escherichia coli undergo acid adaptation wherein there is an induction of glutamate, arginine and lysine decarboxylases and RpoS-dependent oxidative systems, which confers acid tolerance. This study examined the activities of SDH, ICD, MDH and G6PD in acid shocked cells of Escherichia coli DH5á and Escherichia coli W3110 subjected to pH 3, 4, and 5 by different types of acidification, viz. external (using 0.1N HCl), external along with the monensin (1ìM) and cytoplasmic (using 20 mM sodium benzoate), for different time periods. This study highlights new insights into activities of isocitrate, succinate, malate and glucose-6- phosphate dehydrogenases, coupled to the electron transport chain by the reducing power, as yet another system possessed by Escherichia coli as an armor against harsh acidic environments. The activities of dehydrogenases in acid shocked cells were monitored at intervals of 1, 2, 3 and 4 h at different pH and then compared with their activities at pH 7 (taken as control). The results show that an exposure to acidic environment (pH 3, 4 and 5) for a short period of time increased the activities of these dehydrogenases in all types of acidification except cytoplamic acidification used in the current study. In cytoplasmic acidification the activities of all dehydrogenases decreased at pH 3, 4 and 5. On external acidification along with monensin, activities of dehydrogenases increased further as compared to normal external acidification because monensin as an uncoupler destroys the electrical potential of proton motive force. Cells exposed to pH 3 for 2 h had the highest acid tolerance in external acidification with or without monensin. It was also found that activity of glucose-6-phosphate dehydrogenase remained unchanged at low pH. This result suggest that, in a low pH environment, metabolic flux in E. coli increases through TCA cycle and remains unaffected through the pentose phosphate pathway. This increase in metabolic flux through TCA cycle, during oxidative phosphorylation, cause electrons from NADH or FADH2 to pass onto O2 through the electron transport chain located in the plasma membrane of the microorganism, leading to the pumping of protons out of the cytoplasm and thus maintaining pH homeostasis. Keywords: Dehydrogenases, electron transport chain, acid tolerance, external acidification, internal acidification and intracellular pH.