The lysine decarboxylase CadA protects Escherichia coli starved of phosphate against fermentation acids

Conflicting results have been reported for the rate and extent of cell death during a prolonged stationary phase. It is shown here that the viability of wild-type cells (MG1655) could decrease >or=10(8)-fold between days 1 and 14 and between days 1 and 6 of incubation under aerobic and anaerobic phosphate (P(i)) starvation conditions, respectively, whereas the cell viability decreased moderately under ammonium and glucose starvation conditions. Several lines of evidence indicated that the loss of viability of P(i)-starved cells resulted primarily from the catabolism of glucose into organic acids through pyruvate oxidase (PoxB) and pyruvate-formate lyase (PflB) under aerobic and anaerobic conditions, respectively. Weak organic acids that are excreted into the medium can reenter the cell and dissociate into protons and anions, thereby triggering cell death. However, P(i)-starved cells were efficiently protected by the activity of the inducible GadABC glutamate-dependent acid resistance system. Glutamate decarboxylation consumes one proton, which contributes to the internal pH homeostasis, and removes one intracellular negative charge, which might compensate for the accumulated weak acid anions. Unexpectedly, the tolerance of P(i)-starved cells to fermentation acids was markedly increased as a result of the activity of the inducible CadBA lysine-dependent acid resistance system that consumes one proton and produces the diamine cadaverine. CadA plays a key role in the defense of Salmonella at pH 3 but was thought to be ineffective in Escherichia coli since the protection of E. coli challenged at pH 2.5 by lysine is much weaker than the protection by glutamate. CadA activity was favored in P(i)-starved cells probably because weak organic acids slowly reenter cells fermenting glucose. Since the environmental conditions that trigger the death of P(i)-starved cells are strikingly similar to the conditions that are thought to prevail in the human colon (i.e., a combination of low levels of P(i) and oxygen and high levels of carbohydrates, inducing the microbiota to excrete high levels of organic acids), it is tempting to speculate that E. coli can survive in the gut because of the activity of the GadABC and CadBA glutamate- and lysine-dependent acid resistance systems.