The characters on carbon utilization of Escherichia coli NZN111 which over-expresses malic enzyme were investigated.The cell density,metabolite concentration,NADH amount and oxidoreduction potential(ORP)during the fermentation of E.coli NZN111 with different reduced carbon sources were determined in 3 L fermentor.The results showed that when the mannitol,glucose and sodium gluconate were used as sole carbon source,the succinic acid yields were 99.7%,83.0% and 40.5%,respectively.More NADH would be produced when the carbon source with higher reducibility was used,and the NADH amount was gradually decreased during fermentation.At the end of fermentation,the ORP were-358 mV,-367 mV and-384 mV respectively for mannitol,glucose and sodium gluconate used as sole carbon sources.It was indicated that the stronger the reducibility of the carbon source was,the more NADH was produced,and at the end of the fermentation,the more the succinic acid yield was,the less the by-product acetate was produced and the higher the ORP was.

Succinic acid is regarded as C4 platform chemical, which has a huge potential market with wide applications in biopolymer, food, medicine, and so on. Production of succinic acid by microorganism fer-mentation has attracted much attention in recent times. By fermentation, succinic acid could be produced from renewable resources in place of petroleum with low cost and low pollution; especially CO2 could be fixed by microorganism to form the product, which is a promising way to deal with the green gas. Some E. coli have been chosen as good succinic acid producing strains because of their clear genetic backgrounds, convenience to be modified and controlled, and good growth properties with low nutrients requirements. The progress of genetic engineering strategy and metabolic engineering technology for construction of succinic aicd producing recombinant E. coli has been introduced in this paper with the discussion of the future re-search in this area.

The effects of different carbon sources added to aerobic medium on cell growth,enzyme activity and metabolite distribution was investigated in dual-phase fermentations.The results showed that both cell density and enzyme activity were enhanced by adding 4mmol/L glucose or 12,54,80mmol/L sodium acetate to aerobic medium,respectively.Then centrifugated cells that had grown aerobically in different conditions were transferred to anaerobic fermentation,the enzyme activity and metabolite distribution changed.To analyze the anaerobic enzyme activity and metabolite distribution,it was concluded that during the anaerobic fermentation of Escherichia coli NZN111 overexpressed malic enzyme,PEP carboxykinase(PCK)was the key enzyme of succinate production,pyruvate kinase(PYK)was associated to the accumulation of by-product pyruvate,and isocitrate lyase(ICL)have some influence on the concentration of succinate.The yield of succinate in anaerobic stage could reach 89.0%,which was 16.6% higher than that of the control by adding 80mmol/L sodium acetate as carbon source in aerobic medium.

The effect of redox potential regulation on the metabolites distribution in succinic acid production by anaerobic fermentation of Actinobacillus succinogenes was studied. In the serum bottle fermentation,potassium ferricyanide and dithiothreitol which did not have inhibition on fermentation, were selected as the oxidizing agent and reducing agent respectively. Potassium ferricyanide and dithiothreitol were used to control redox potential from -100 mV to -450 mV in 3 L fermenter. It was shown that the optimum redox potential or oxidoreduction potential (ORP) for cell growth and succinic acid production was -350 mV. Comparing with the fermentation without redox potential regulation,the productivity of succinic acid and the ratio of succinic acid to acetic acid enhanced from 0.75 g/(L·h) to 1.18 g/(L·h) and 2.5 to 3.9 respectively.

Plackett-Burman design was used to evaluate the effects of eleven factors for succinic acid production by Escherichia coli NZN111(sfcA).The results showed the following key factors: sodium acetate,magnesium carbonate,inoculum size,time length of adding IPTG and incubation period.Based on these results,response surface methodology(RSM) was applied for further optimizing these parameters in order to find out the optimum concentration levels.The concentration of succinic acid could reach 15.64g/L,compared to the initial 11.84 g/L,and the yield was enhanced from 53.8 % to 71.1 % with the initial glucose concentration of 22 g/L,1 g/L of acetate,17 g/L of magnesium carbonate,10 % of inoculum size,IPTG initially added and incubation period of 76 h.