The present state of the art knows a number of diverse processes for producing L-threonine by virtue of submerged cultivation of producer strains of such species as Brevibacterium flavum, Escherichia coli, Corynebacterium acetoscidophilum, Proteus rettgeri, Serratia marcescensi, Aerobacter aerogenes, Corynebacterium glutamicum, grown on nutrient media, containing such carbon sources as glucose, fructose, acetic acid, ethanol and doped with some vitamins and aminoacids, or substrates containing these, as well as incorporating nitrogen sources and indispensable mineral salts (cf. British Pat. Nos. 1,223,470; 1,260,995; 1,286,208; French Pat. Nos. 1,573,433; 1,580,549; 1,579,835; 1,603,855; Patent of FRG No. 2,044,907).
The best yield of L-threonine has been achieved from mutant strains of Br.flavum producing up to 18 g/l of L-threonine on a glucose medium (cf. Agr. Biol. chem., 1973, 37, 653), up to 40 g/l of L-threonine on an acetic-acid medium (cf. British Pat. No. 1,260,995), and up to 33.8 g/l of L-threonine on an ethanol medium (cf. British Pat. No. 1,286,208). In all these cases use has been made of mutant strains resistant to .alpha.-amino-.beta.-hydroxy-valeric acid.
One prior-art process for producing L-threonine is known to consist in submerged cultivation of mutant strains of Escherichia coli on nutrient media, containing carbohydrates, nitrogen sources, mineral salts and doped with pure aminoacids and vitamins, or protein mass hydrolyzates containing these. A maximum yield of 10.4 g/l of L-threonine has been obtained on an enzymatic medium for 96 hours of fermentation (cf. British Pat. No. 1,223,470; U.S. Pat. No. 3,711,375; French Pat. Nos. 1,551,414 and 1,580,545).
The mutant strains made use of in the above-mentioned processes exhibit the need for isoleucine, or for isoleucine and methionine (cf. British Pat. No. 1,223,470), or for diaminopimelic acid (cf. U.S. Pat. No. 3,711,375; French Pat. No. 1,551,414).
The processes discussed above are featured, however, by a low level of L-threonine accumulation, particularly on carbohydrate media, as well as a prolonged fermentation period (96 to 120 hours).
One more process for producing L-threonine is known in the art by way of submerged cultivation of an L-threonine producer on a nutrient medium, containing carbon and nitrogen sources and some mineral salts. Used as the producer of L-threonine are some polyauxotrophic E.coli mutants, such as the E.coli strains ATCC 21272, ATCC 21148, and ATCC 21149, whereas glucose or fructose is applied as a source of carbon. These strains exhibit the need for diaminopimelic acid, isoleucine, or methionine. Thus, for instance, the E.coli strain ATCC 21272 needs diaminopimelic acid, methionine, isoleucine, while the strain ATCC 21148 needs diaminopimelic acid and methionine, and the strain ATCC 21149 needs diaminopimelic acid alone. In addition, the cultivation of these strains involves, apart from the above growth factors, also lysin. The cultivation is carried out at 20.degree. at 40.degree. C. When cultivated on a medium, containing 7.5 percent fructose the abovesaid strains produce L-threonine, its concentration in the culture medium first rising, but then starts decreasing with time due to degradation. In order to extend the period of L-threonine accumulation and prevent its concentration from diminishing, some antibiotics are used, such as streptomycin, tetracycline, kanamycin, polymyxin, or their mixtures.
When streptomycin is added 40 hours after the beginning of the fermentation, the latter is prolonged up to 120 hours, and the L-threonine accumulation level amounts to 13.2 g/l, which is the case with the cultivation of the E-coli strain ATC 21148 grown on a nutrient medium, containing 7.5 percent fructose and needing diaminopimelic acid and methionine. The amount of the antibiotic added to the nutrient medium ranges within 10 to 1000 mg/l (cf. French Pat. No. 1,579,835).
This process suffers largely from a low average rate of accumulation of the end product in the course of fermentation, which is as low as within 0.15 to 0.20 g/l per hour, thus resulting in a low L-threonine accumulation level and an increased duration of the fermentation process. Moreover, application of this process involves as introduction of some additional components into the medium, such as diaminopimelic acid, methionine or isoleucine, which the producers of the process exhibit the need for.