Enhancing sodium chloride tolerance of Escherichia coli
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The primary goal of this research is to enhance the sodium chloride tolerance of E. coli in order to prolong the fermentation time and improve organic acid production. One means examined was to overexpress nhaA encoding a native Na+/H+ antiporter, which led to severely curtailed microbial growth. Adaptive evolution was then employed to generate NaCl-tolerant random mutants. Four E. coli mutants with enhanced ability of NaCl tolerance, designated ALS1184–ALS1187, were isolated after seventy three days of transfer. Several physiological parameters of MG1655 and ALS1187 were calculated and compared using glucose-limited chemostats. Strains ALS1317 (MG1655 pflB) and ALS1318 (ALS1187 pflB) were constructed for lactate accumulation. ALS1318 generated 76.2 g/L lactate compared to 56.3 g/L lactate produced by ALS1317. The same evolutionary approach used on ALS929, a pyruvate-generating E. coli strain, resulted in four isolates with greater NaCl tolerance, but did not improve pyruvate production under comparable conditions.