Abstract:
Bacterial strains of Corynebacterium glutamicum isolated from various natural environments were initially selected for their capacity to accumulate glutamic acid and then subjected to EMS (Ethyl Methan Sulfonate), chemical mutagenesis.
Strains were then selected for amino acid auxotrophies and their capacities to produce other amino acids (notably lysine) were examined.
A strain initially isolated from bird droppings and having a double auxotrophy for threonine and methionine was thus selected and retained for characterization as to its capacity to produce significant amounts of lysine.
This strain grew at rates approximately half that of the parent strain but accumulated lysine as a primary metabolite throughout growth. Final concentrations of approximately 50 g/L lysine could be produced on glucose medium and this fermentation limit was shown to be directly related to the end concentration of lysine.
Glucose was shown to be the best carbon source though sucrose showed similar capacity albeit with slightly diminished rates of sugar consumption and lysine accumulation. Acetate gave only relatively poor fermentation performance.
Stoichiometric metabolic flux analysis can be examined to see how flux through central metabolism influences amino acid yields. When used here such an analysis shows that the lysine yields, based on experimentally determined rates of sugar consumption, lysine production and specific growth rate optimization, and were close to the maximal levels which can be expected. Indeed carbon recovery yields are close to 30% which is basically the yields predicted for the strain when growth rate and substrate consumption rates are used. Such calculations of stoichiometric flux analysis need always to fix certain variables: substrate uptake rate was assumed to be constant at all theoretical growth rates tested (value observed in that mutant strain).
If yields are acceptable they could be further improved by appropriate fed-batch
fermentation strategies in which growth rate was further diminished assuming that sugar uptake can be maintained. However, further development is necessary to improve the final concentrations of lysine which are still relatively low compared to currently exploited industrial strains.
They are however rather promising for a strain having undergone a single round of
mutation. The strain is therefore a relatively efficient lysine producer whose biotechnological value could be further improved by additional mutational selection strategies to facilitate a non-OGM source of feed for improving poultry production.
