Read this article to learn about the features and methods of genetic improvement of strains.
In general, the wild strains of microorganisms produce low quantities of commercially important metabolites, although the yield can be increased by optimizing the fermentation conditions. The potentiality of the metabolite formation is genetically determined. Therefore, genetic improvements have to be made and new strains developed for any substantial increase in product formation in a cost-effective manner.
There are strain development programmes (mutation and recombination) to increase the product yield by 100 times or even more. The nature of the desired product determines the success associated with strain improvement. For example, if alterations in one or two genes (i.e. one or 2 key enzymes) can improve the product yield, it is simpler to achieve the target.
This type of approach is sometimes possible with primary metabolites. As regards the secondary metabolites, the product formation and its regulation are quite complex. Hence, several genetic modifications have to be done to finally produce high-yielding strains.
Features of Genetic Improvement:
Ideally speaking, the improved strains should possess the following characteristics (as many as possible) to finally result in high product formation:
1. Shorter time of fermentation
2. Capable of metabolizing low-cost substrates
3. Reduced O2 demand
4. Decreased foam formation
5. Non-production of undesirable compounds
6. Tolerance to high concentrations of carbon or nitrogen sources
7. Resistant to infections of bacteriophages.
It is always preferable to have improved strains of microorganisms which can produce one metabolite as the main product. In this way, the production can be maximised, and its recovery becomes simpler. Through genetic manipulations, it has been possible to develop strains for the production of modified or new metabolites which are of commercial value e.g. modified or newer antibiotics.
The major limitation of strain improvement is that for most of the industrially important microorganisms, there is lack of detailed information on the genetics, and molecular biology. This hinders the new strain development.
Methods of Strain Development:
There are two distinct approaches for improvement of strains-mutation, recombination and recombinant DNA technology.
1. Mutation:
Any change that occurs in the DNA of a gene is referred to as mutation. Thus, mutations result in a structural change in the genome. Mutations may be spontaneous (that occur naturally) or induced by mutagenic agents.
The spontaneous mutations occur at a very low frequency, and usually are not suitable for industrial purposes. Mutations may be induced by mutagenic agents such as ultraviolet light, various chemicals (nitrous oxide, nitrosoguanidine, and hydroxylamine). Site-directed mutagenesis is also important for strain improvement.
Selection of Mutants:
Selection and isolation of the appropriate mutant strains developed is very important for their industrial use. Two techniques commonly employed for this purpose are briefly described.
Random screening:
The mutated strains are randomly selected and checked for their ability to produce the desired industrial product. This can be done with model fermentation units. The strains with maximum yield can be selected. Random screening is costly and tedious procedure. But many a times, this is the only way to find the right strain of mutants developed.
Selective isolation of mutants:
There are many methods for selective isolation of improved strains:
1. Isolation of antibiotic resistant strains:
The mutated strains are grown on a selective medium containing an antibiotic. The wild strains are killed while the mutant strains with antibiotic resistance can grow. Such strains may be useful in industries.
2. Isolation of antimetabolite resistant strains:
Antimetabolites which have structural similarities with metabolites can block the normal metabolic pathways and kill the cells. The mutant strains resistant to antimetabolites can be selected for industrial purposes. In the Table 19.5, a selected list antimetabolites used for screening the metabolites is given.
3. Isolation of auxotrophic mutants:
An auxotrophic mutant is characterized by a defect in one of the biosynthetic pathways. As a result, it requires a specific compound for its normal growth. For instance, tyr mutants of Corynebacterium glutamicus require tyrosine for their growth while they can accumulate phenylalanine. The isolation of such mutants can be done by growing them on a complete agar medium that can specifically support the biochemically defective mutant.
2. Genetic Recombination:
The strain improvement can be made by combining genetic information from two genotypes, by a process called genetic recombination. The recombination can be brought out by transformation, transduction, conjugation and protoplast fusion.
There are many advantages of genetic recombination:
1. By crossing high product yielding mutant strains with wild-type strains, the fermentation process can be further increased.
2. Different mutant strains with high-yielding properties can be combined by recombination.
3. There is gradual decline in the product yield after each stage of mutation, due to undesirable mutations. This can be prevented by using recombination.