Food-borne bacteria pose a threat to global health. In 1982, the E. coli strain O157: H7 caused a global pandemic. In 2011, the E. coli strain O104: H4 caused an outbreak in Europe, killing 18 people and infecting one in every 500 people. In order to be able to pass through the stomach with a pH of about 2 and survive in other acidic environments, E. coli has formed sophisticated acid-tolerant systems (ARs). Among the three known ARs, the functional mechanism of AR1 is still unclear. In contrast, the molecular mechanisms of AR2 and AR3 have been more deeply resolved. AR2 contains an amino acid reverse transport protein GadC, which is responsible for the exchange of extracellular L-glutamic acid (Glu) with intracellular Î³-aminobutyric acid (GABA). The two Glu decarboxylases GadA and GadB convert Glu to GABA. Similar to AR2, AR3 also has two components: anti-transporter AdiC and arginine deacidase AdiA. A complete cycle of AR2 or AR3 can expel protons in the cytoplasm to the extracellular environment, thereby increasing the intracellular pH and promoting the survival of bacteria in an acidic environment.
A comprehensive understanding of bacterial AR is of great significance for effective clinical prevention and treatment. Because all food-borne pathogens must pass through extremely acidic stomachs, it is extremely important to understand the survival mechanism of bacteria in an environment with a pH of 2-3. Currently, researchers do not know enough about these mechanisms. In this study, the researchers identified a new acid-tolerant system in E. coli and confirmed that it depends on glutaminase YbaS and the amino acid reverse transport protein GadC. This YbaS and GadC can be activated by acidic pH, and can only function properly when the pH is less than or equal to 6.0. By absorbing L-glutamine (Gln), E. coli uses YbaS to convert it to L-glutamic acid (Glu) with the release of gaseous ammonia. Free ammonia neutralizes protons, resulting in increased intracellular pH in an acidic environment. GadC is responsible for the exchange of extracellular Gln and intracellular Glu. Through this acid-resistant system, E. coli is ensured to survive in a very acidic environment.
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