Since Robert Knock, the Nobel Prize winner, proved that has a positive influence on human metabolism and there is no risk of its accumulation in the human body, it has been used in food industries. Succinic acid is an intermediate of the tricarboxylic acid (TCA) cycle and one of the fermentation end-products of anaerobic metabolism. Thus, it is synthesized in almost all microbial, plant and animal cells. Those organisms suitable for the efficient production of succinic acid can be categorized into fungi and bacteria.
Many researchers have made tremendous efforts to develop a biological process for the production of succinic acid by employing fungi such as Aspergillus niger, Aspergillus fumigatus,Byssochlamys nivea, Lentinus degener, Paecilomyces varioti, Penicillium viniferum and yeast Saccharomyces cerevisiae.
These organisms produce succinic acid as a metabolic byproduct under aerobic and/or anaerobic conditions.S. cerevisiae has been best studied among them to achieve high concentration of succinic acid in the manufacture of wine. A series of its mutant strains were developed by the inactivation of undesired genes, and some of them showed the
increased levels of succinic acid compared with the wild type strain.A. niger has been recognized as a very important organism for the production of various organic acids, especially citric acid and gluconic acid. This organism produces more than 78 g l-1 of citric acid with the yield of 65% (w/w) on sucrose. Furthermore,it shows an ability to utilize various carbon sources with a good yield (115%, w/w) on rapeseed oil. Recently,the central carbon metabolism of this organism and its metabolic network were deciphered by combining genomic, biochemical and physiological information. Based on them, a stoichiometric model composed of 284 metabolites and 335 reactions was constructed. Simulation of this stoichiometric model suggested that this organism can produce 1.5 mol succinic acid from 1 mol glucose under microaerobic condition [31]. However, the use of fungi has been mostly limited to the manufacture of food and beverages due to the difficulties in fermentation, separation and purification as well as low productivities.
Only few Gram-positive bacteria like Corynebacterium glutamicum and Enterococcus faecalis have been studied for succinic acid production. Several engineered C. glutamicum strains were created by disruption and replacement of genes, and their optimal culture conditions were developed. It was possible to increase the succinic acid production rate seven times and the glucose consumption rate five times under oxygen deprived condition. A two-step succinic acid production process was developed, in which fumaric acid obtained from the fermentation of glucose and rice bran using Rhizopus sp. is subsequently converted to succinic acid by E. faecalisRKY1. The yield of fumaric acid conversion to succinic acid was 95% (w/w) and the productivity was 2.2 g l-1 h-1. However, it should be noted that the yield of fumaric acid on glucose and the fumaric acid productivity
in the first step were quite low at the levels of 0.5 g g-1 and 0.21 g l-1 h-1, respectively, limiting its commercialization.
The metabolic pathways leading to the synthesis of succinic acid are diverse. Some bacteria mainly utilize the phosphoenolpyruvate (PEP) carboxylation reaction, while others use
multiple pathways to form succinic acid. Many different succinic acid producing Gram-negative bacteria have been isolated in various anaerobic environments such as domestic sludge, cattle waste, rice paddy, marine shipworm, mouth of dog, rumen and gastro-intestines. To date, the bacteria isolated from the rumen, including A. succinogenes and M. succiniciproducens, are the best candidates for succinic acid production as they produce succinic acid as a major fermentation product. This is most likely due to that the rumen is a highly efficient organ providing an environment to produce succinic acid. The rumen is a unique microbial ecosystem found in many species of herbivorous mammals known as ruminants. The primary role of the rumen is to allow pre-gastric digestion of various polysaccharide materials, which is mediated by a great diversity of rumen microorganisms, consisting of 109–1010 bacterial, 105–106 protozoan and 103–104 fungal cells ml-1 of rumen fluid. The production of C4 dicarboxylic acids in the rumen reduces energy
loss associated with methanogenesis (30–40 mol% of CH4 is present in the ruminal gas) by increasing the amount of metabolizable energy available to the animal in the form of propionic acid. Although the C4 dicarboxylic compounds, such as oxaloacetic, malic, fumaric and succinic acids are not detected in the ruminal fluid, large amounts of these acids are produced by CO2 fixation reactions, using 60–70 mol% of CO2 present in the ruminal gas. The major C3 compounds in the cell used for carboxylation reaction are PEP and pyruvate. In particular, succinic acid is converted to propionic acid, which can account for 20% (w/w) of total volatile fatty acids (VFAs) in the rumen, by succinic acid utilizing bacteria such as Veillonella parvula,Selenomonas ruminantium and Succiniclasticum ruminis. Propionic acid produced this way is absorbed through the rumen wall for subsequent oxidation to provide energy and biosynthetic precursors for the animals. Therefore, it is reasonable to think that some microorganisms present in the rumen will be a good succinic acid producer. Here, we review the metabolic characteristics and fermentation performances of two rumen bacteria A. succinogenes and M. succiniciproducens along with two other good candidate bacteria A. succiniciproducens and recombinant E. coli. A