Adaptation to acid stress is an important factor in the transmission of intestinal microbes. The enterobacterium Escherichia coli uses a range of physiological, metabolic, and proton-consuming acid resistance mechanisms in order to survive acid stresses as low as pH 2.0. The physiological adaptations include membrane modifications and outer membrane porins to reduce proton influx and periplasmic and cytoplasmic chaperones to manage the effects of acid damage. The metabolic acid resistance systems couple proton efflux to energy generation via select components of the electron transport chain, including cytochrome bo oxidase, NADH dehydrogenase I, NADH dehydrogenase II, and succinate dehydrogenase. Under anaerobic conditions the formate hydrogen lyase complex catalyzes conversion of cytoplasmic protons to hydrogen gas. Finally, each major proton-consuming acid resistance system has a pyridoxal-5'-phosphate-dependent amino acid decarboxylase that catalyzes proton-dependent decarboxylation of a substrate amino acid to product and CO2, and an inner membrane antiporter that exchanges external substrate for internal product.