| Approach |
Chief feature |
Example/Remark |
| A. Mutant Selection : Types |
|
The main approach to strain improvement; produces new alleles of existing genes |
| 1. Spontaneous Mutations |
Occur without any treatment with a mutagen |
Used in, the initial stages of strain improvement; also for maintenance of improved strains |
| 2. Induced Mutations |
Induced by chemical (mainly) or physical mutagens |
Mutagenesis followed by selection; several cycles employed |
| 3. Major Mutations |
Affect the pattern of metabolite production |
Production of 6-demethyl tetracycline in place of tetracycline by S. aureofaciens |
| 4. Minor Mutations |
Affect the rate metabolite production |
Small gains in each cycle of selection; substantial improvement after several cycles |
| B. Mutant Selection :Strategies |
|
|
| 1. Auxotrophic mutants |
Defective biosynthesis of a biochemical |
Enhanced production of an amino acid, e.g., phe mutants accumulate tyrosine |
| 2. Analogue resistant mutants |
Feedback insensitive enzymes |
Overproduction of metabolites, e.g., amino acids by C. glutamicus |
| 3. Revertants of nonproducing mutants |
|
Some mutants are high producers, e.g., chlortetracycline by S. viridifaciens |
| 4. Revertants of auxotrophic mutants |
|
Some are high produces, e.g. chlortetracycline by S. viridifaciens |
| 5. Resistance to the antibiotic produced by the organism itself |
|
Increased production, e.g., chlortetracycline by S. aureofaciens |
| C. Recombination |
|
Produces new combinations of existing alleles |
| 1. Sexual reproduction |
Conjugation; fusion of gametes |
Some bacteria and Actinomycetes; fungi and yeast |
| 2. Heterokaryosis |
Nuclear fusion followed, by mitotic recombination and mitotic reduction |
Fungi |
| 3. Protoplast fusion |
Protoplasts produced by lytic enzymes fusion by PEG, recombinant recovery |
Bacteria, Actinomycetes, fungi; quite; successful |
