Importance of balanced intracellular formate levels for pH(i) and CO(2) homeostases during mixed-acid fermentation.

A Escherichia coli strain expressing the FocA N209 formate-export variant shows increased intracellular pH and reduced ATP during fermentative growth.

• Why it matters: Balanced intracellular formate is required for CO2 generation by formate hydrogenlyase (FHL-1) and for maintaining pH homeostasis and energy metabolism.
• What they did: The study compared the FocA N209 mutant with the parental strain during anaerobic mixed-acid fermentation, measuring pH(i), ATP, and fermentation-product profiles, including bicarbonate supplementation.
• The result: Bicarbonate partially restored anaerobic growth but only marginally lowered pH(i) and left ATP low; formate loss was not compensated, and lactate/succinate were only partially rescued while ethanol increased.