1. Field of the Invention
The present invention relates to a method for producing L-glutamic acid by fermentation. L-glutamic acid is widely used as a raw material in the production of seasonings and so forth.
2. Description of the Related Art
L-glutamic acid is mainly produced by fermentation using an L-glutamic acid-producing bacterium of the so-called coryneform bacterium belonging to the genus Brevibacterium, Corynebacterium or Microbacterium or mutant strains thereof. Moreover, methods utilizing a microorganism belonging to the genus Bacillus, Streptomyces, Penicillium, Pseudomonas, Arthrobacter, Serratia, Candida, or Aerobacter aerogenes (currently Enterobacter aerogenes), a mutant strain of Escherichia coli, or the like are known. Furthermore, also known are methods of producing L-glutamic acid using a microorganism belonging to the genus Klebsiella, Erwinia, or Pantoea (U.S. Pat. No. 6,197,559), and methods of producing L-glutamic acid using an Enterobacter bacterium (U.S. Pat. No. 6,331,419).
Furthermore, various techniques for improving L-glutamic acid-producing ability by enhancing activities of L-glutamic acid biosynthetic enzymes through the use of recombinant DNA techniques have been disclosed. For example, it was reported that the introduction of a gene encoding citrate synthase derived from Escherichia coli or Corynebacterium glutamicum was effective for enhancing L-glutamic acid-producing ability in bacterium belonging to the genus Corynebacterium or Brevibacterium (Japanese Patent Publication (JP 7-121228 B). In addition, JP 61-268185 A discloses a cell harboring recombinant DNA containing a glutamate dehydrogenase gene derived from Corynebacterium bacteria. Furthermore, JP 63-214189 A discloses a technique for increasing L-glutamic acid-producing ability by amplifying genes encoding glutamate dehydrogenase, isocitrate dehydrogenase, aconitate hydratase, and a citrate synthase.
L-glutamic acid production has been considerably increased by the aforementioned breeding of microorganisms or improving production methods. However, in order to respond to increased demand in the future, the development of methods which provide more efficient production of L-glutamic acid at a lower cost are still necessary, and therefore, still represent a need in the art.
Methods for L-glutamic acid fermentation while precipitating L-glutamic acid, which accumulates in culture broth, have been developed (EP 1078989 A). Because the usual L-glutamic acid-producing bacteria cannot grow under acidic conditions, L-glutamic acid fermentation was conventionally performed under neutral conditions. Contrary to such conventional techniques, microorganisms which could produce L-glutamic acid under acidic conditions were screened, and it has been reported that L-glutamic acid can be produced and accumulated in the medium while precipitating the L-glutamic acid by culturing the obtained microorganism (Enterobacter agglomerans) in a liquid medium in which the pH is controlled so that L-glutamic acid is precipitated.
Furthermore, methods are known for producing L-glutamic acid by culturing such an L-glutamic acid-producing bacterium that can grow under acidic conditions as described above in a medium having a total content of organic acids that inhibit growth of the bacterium in an amount that does not inhibit the growth of the bacterium (European Patent Application Laid-open No. 1233070) and for producing L-glutamic acid by culturing such a bacterium as described above at a first pH optimal for growth of the microorganism and then culturing the bacterium at a second pH optimal for L-glutamic acid production by the microorganism and lower than the first pH (European Patent Application Laid-open No. 1233068). Furthermore, a method is known for producing and accumulating L-glutamic acid in a medium while precipitating the L-glutamic acid in the medium, wherein crystals of L-glutamic acid are made to exist in the medium while L-glutamic acid concentration in the medium is lower than the concentration at which natural crystallization of L-glutamic acid occurs (European Patent Application Laid-open No. 1233069).