The Lysine Decarboxylase CadA Protects Escherichia coli Starved of Phosphate against Fermentation Acids

Laboratoire de Chimie Bactérienne, Centre National de la Recherche Scientifique, 13009 Marseille, France

^* To whom correspondence should be addressed. Email: moreau{at}ibsm.cnrs-mrs.fr.

	Abstract

Conflicting results have been reported about the rate and extent of cell death during prolonged stationary phase. It is shown here that the viability of wt cells (MG1655) could drop 10⁸-fold between days 1 and 14, and days 1 and 6 of incubation under aerobic and anaerobic, phosphate (Pi) starvation conditions, respectively, whereas cell viability decreased moderately under ammonium and glucose starvation conditions. Several lines of evidence indicate that the loss of viability of Pi-starved cells primarily resulted from the catabolism of glucose into organic acids through pyruvate oxydase (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, Pi-starved cells were efficiently protected through the activity of the inducible glutamate-dependent acid resistance system GadABC. 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 Pi-starved cells to fermentation acids was markedly increased as a result of the activity of the inducible lysine-dependent acid resistance CadBA 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 E. coli since the protection of E. coli challenged to pH 2.5 by lysine is much weaker than that by glutamate. CadA activity was favored in Pi-starved cells probably because weak organic acids slowly reenter cells fermenting glucose. The environmental conditions that trigger the death of Pi-starved cells being strikingly similar to the conditions that are thought to prevail in the human colon (i.e., a combination of low levels of Pi 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 glutamate- and lysine-dependent acid resistance systems GadABC and CadBA.