Figure 1

A schematic of metabolic and gene regulation network model of the production of lactic acid. The LacS, LacZ and LDH represent lactose permease, β-galactosidase, and lactate dehydrogenase respectively. When the extracellular pH reduces to 5.5, the pH-induced promoter (we chose rcfB promoter as an example) will trigger the express of LacR. The accumulation of LacR will combine with the operator sequence upstream of lacS gene, inhibiting the expression of LacS and LacZ [17].