The ability to grow at moderate acidic conditions (pH 4.0-5.0) is important to Escherichia coli colonization of the host's intestine. Several regulatory systems are known to control acid resistance in E. coli, enabling the bacteria to survive under acidic conditions without growth. Here, we characterize an acid-tolerance response (ATR) system and its regulatory circuit, required for E. coli exponential growth at pH 4.2. A two-component system CpxRA directly senses acidification through protonation of CpxA periplasmic histidine residues, and upregulates the fabA and fabB genes, leading to increased production of unsaturated fatty acids. Changes in lipid composition decrease membrane fluidity, F 0 F 1 -ATPase activity, and improve intracellular pH homeostasis. The ATR system is important for E. coli survival in the mouse intestine and for production of higher level of 3-hydroxypropionate during fermentation. Furthermore, this ATR system appears to be conserved in other Gram-negative bacteria.

Poly(3-hydroxybutyrate) (PHB) is a biodegradable and biocompatible thermoplastic, and synthesized from the central metabolite acetyl-CoA. The acetyl-CoA synthesis from glucose presents low atomic economy due to the release of CO 2 in pyruvate decarboxylation. As ethanol and acetate can be converted into acetyl-CoA directly, they were used as carbon source for PHB production in this study. The reductase mutant AdhE A267T/E568K was introduced into Escherichia coli to enable growth on ethanol, and acetate utilization was improved by overexpression of acetyl-CoA synthetase ACS. Comparison of the PHB production using glucose, ethanol or acetate as sole carbon source showed that the production and yield from ethanol was much higher than those from glucose and acetate, and metabolome analysis revealed the differences in metabolism of glucose, ethanol and acetate. Furthermore, other acetyl-CoA derived chemicals including 3-hydroxypropionate and phloroglucinol were produced from those three feedstocks, and similar results were achieved, suggesting that ethanol could be a suitable carbon source for the production of acetyl-CoA derivatives.

Background: Acetyl-CoA is a fundamental metabolite in Escherichia coli, and also a precursor for biosynthesis of chemicals and materials suitable for multiple applications. The acetyl-CoA synthesis route from glucose presents low atomic economy due to the release of CO2 in pyruvate decarboxylation reaction. Because ethanol and acetate, both ordinary and inexpensive chemicals, can be converted into acetyl-CoA directly, they could be alternative substrates for production of acetyl-CoA derivatives. Results: In this study, the bifunctional reductase AdhE mutant (A267T/E568K), which converts ethanol into acetyl-CoA, was used to enable E. coli to grow on ethanol, and AMP-forming acetyl-CoA synthetase ACS was employed to enhance the ability of E. coli to utilize acetate. Several products derived from acetyl-CoA, including polyhydroxybutyrate, 3-hydroxypropionate, and phloroglucinol, were produced from glucose, ethanol, and acetate, respectively, by engineered E. coli strains. Compared with glucose and acetate, the strains grown on ethanol presented the highest production and yield of carbon source, and metabolome analysis revealed the reasons of high yield from ethanol. Conclusions: The conversion of ethanol into acetyl-CoA presents high atomic economy along with generation of reducing power, and the yield of target chemical from ethanol is much higher than those from glucose and acetate. All these results suggested that ethanol could be a suitable carbon source for production of acetyl-CoA derived bioproducts.