Ko Y H, Gross R A
Department of Chemistry, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854, USA.
Biotechnol Bioeng. 1998 Feb 20;57(4):430-7. doi: 10.1002/(sici)1097-0290(19980220)57:4<430::aid-bit6>3.0.co;2-n.
Bacillus licheniformis ATCC 9945a is one of the bacterial strains that produce gamma-poly(glutamic acid) (gamma-PGA). The use of carbohydrate medium components for gamma-PGA production was explored. Cells were grown in shake flasks or in controlled pH fermentors using medium formulations that contain different carbon sources. During the cultivations, aliquots were removed to monitor cell growth, carbon utilization, polymer production, and polymer molecular weight. Glucose was a better carbon source than glycerol for cell growth. Furthermore, glucose was utilized at a faster rate than glycerol, citrate, or glutamate. However, by using mixtures of glucose and glycerol in medium formulations, the efficiency of gamma-PGA production increased. For example, by increasing the glycerol in medium formulations from 0 to 40 g/L, the gamma-PGA broth concentration after 96 h increased from 5.7 to 20.5 g/L. Considering that glycerol utilization was low for the glucose/glycerol mixtures studied, it was unclear as to the mechanism by which glycerol leads to enhanced product formation. Cell growth and concomitant gamma-PGA production (12 g/L) at pH 6.5 was possible using glucose as a carbon source if trace amounts (0.5 g/L each) of citrate and glutamate were present in the medium. We suggested that citrate and glutamate were useful in preventing salt precipitation from the medium. In addition, glutamate may be preferred relative to ammonium chloride as a nitrogen source. The conversion of glucose to gamma-PGA by the strain ATCC 9945a was believed to occur by glycolysis of glucose to acetyl-CoA and tricarboxylic acid (TCA) cycle intermediates that were then metabolized via the TCA cycle to form alpha-ketoglutarate, which is a direct glutamate precursor.
地衣芽孢杆菌ATCC 9945a是产生γ-聚谷氨酸（γ-PGA）的菌株之一。研究了用于γ-PGA生产的碳水化合物培养基成分。使用含有不同碳源的培养基配方，将细胞在摇瓶或控制pH的发酵罐中培养。在培养过程中，取出等分试样以监测细胞生长、碳利用、聚合物生产和聚合物分子量。对于细胞生长而言，葡萄糖是比甘油更好的碳源。此外，葡萄糖的利用速率比甘油、柠檬酸盐或谷氨酸盐更快。然而，通过在培养基配方中使用葡萄糖和甘油的混合物，γ-PGA的生产效率提高了。例如，通过将培养基配方中的甘油从0增加到40 g/L，96小时后γ-PGA发酵液浓度从5.7 g/L增加到20.5 g/L。考虑到所研究的葡萄糖/甘油混合物中甘油利用率较低，甘油导致产物形成增强的机制尚不清楚。如果培养基中存在微量（各0.5 g/L）的柠檬酸盐和谷氨酸盐，以葡萄糖作为碳源时，在pH 6.5条件下细胞生长和伴随的γ-PGA生产（12 g/L）是可能的。我们认为柠檬酸盐和谷氨酸盐有助于防止培养基中盐沉淀。此外，相对于氯化铵，谷氨酸盐可能更适合作为氮源。据信，菌株ATCC 9945a将葡萄糖转化为γ-PGA的过程是通过葡萄糖糖酵解为乙酰辅酶A和三羧酸（TCA）循环中间体，然后这些中间体通过TCA循环代谢形成α-酮戊二酸，α-酮戊二酸是谷氨酸的直接前体。
