Strain Improvement

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STRAIN IMPROVEMENT

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•Introduction•Process involved

Selection of natural variantsSelection of induced mutantsThe use of recombination systems

•Important characteristics for strain improvementThe selection of stable strainsThe selection of strains resistant to infectionThe selection of non-foaming strainsThe Selection of strains which are resistant to components

in the mediumThe selection of morphologically favourable strainsThe selection of strains which are tolerant of low oxygen

tensionThe elimination of undesirable Products from a production

strainThe development of strains producing New fermentation

products•Conclusion•Reference

INDEX

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Your logoStrain: A strain is a subgroup of a species with one or more characteristics that distinguish it from other subgroups of the same species. Each strain is identified by a name, number, or letter.

For example: E. coli strain K12, E. coli strain 0157:H7

Introduction:

[Ref: Jacquelyn G. Black (pg no. 242), Tortora (Pg. no. 18)]

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[Ref: www.indianscience.in]

Strain improvement: The science and technology of manipulating and improving microbial strains, in order to enhance their metabolic capacities is known as Strain Improvement.

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Process of strain improvement:

Selection of natural variants

Selection of induced mutants

Use of recombina

nt technology

[Ref: Stanbury, Principles of fermentation technology

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Selection of natural variantsKeywords:

Genetic changes Cell division Variants Mycelial organisms Heterokaryons Homokaryons

[Ref: Stanbury, Principles of fermentation technology

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• The selection of induced mutants synthesizing improved levels of primary metabolites:

Selection of induced mutants

The levels of primary metabolites in micro-organisms are regulated by Feedback control systems.

The major systems involved are feedback inhibition and feedback repression.

[Ref: Stanbury, Principles of fermentation technology

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FIG.1.1 The control of a biosynthetic pathway converting precursor A to end product E via the intermediates B, C and D.

[Ref: Stanbury, Principles of fermentation technology

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Concerted or multivalent feedback control:

FIG.1.2 The control of a biosynthetic pathway by the concerted effects of products D and F on the first enzyme of the pathway.

[Ref: Stanbury, Principles of fermentation technology

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Co-operative feedback control

FIG. 1.3 The control of a biosynthetic pathway by the co-operative control by end products D and F.

[Ref: Stanbury, Principles of fermentation technology

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Cumulative feedback control

--- 50% ---- Inhibition of 50% of the activity of the enzyme

------ Total inhibition of enzyme activity

FIG.1.4 The control of a biosynthetic pathway by the cumulative control of products D and F.

[Ref: Stanbury, Principles of fermentation technology

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Sequential feedback control

FIG.1.5 The control of a biosynthetic pathway by sequential feedback control.

[Ref: Stanbury, Principles of fermentation technology

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Isoenzyme control

Fig.1.6 The control of two isoenzymes (catalysing the conversion of A to B) by end products D and F.

[Ref: Stanbury, Principles of fermentation technology

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•The isolation of mutants which do not produce feedback inhibitors or repressors:

Fig.1.7 Overproduction of primary metabolites by decreasing the concentration of a repressing or inhibiting end product.

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•Examples of the use of auxotrophs for the production of primary metabolites:

FIG. 1.8. The control of the aspartate family of amino acids in C. glutamicum.

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The use of recombination systems for the improvement of industrial micro-organisms• Recombinant DNA techniques – In simple words, rDNA technique can be explained as, bringing together in one organism, genes from several organisms, has the potential for not only increasing yields but also for producing entirely new substances.

• Recombinant DNA technology has resulted in organisms producing compounds which they were not able to produce previously.

[Ref: Stanbury, Principles of fermentation technology]

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The application of the parasexual cycle•Many industrially important fungi do not possess a sexual stage and therefore it would appear difficult to achieve recombination in these organisms. •However, Pontecorvo et al. (1953) demonstrated that nuclear fusion and gene segregation could take place outside, or in the absence of, the sexual organs.

•The process was termed the parasexual cycle and has been demonstrated in the imperfect fungi, A. niger and P. chrysogenum, as well as the sexual fungus A. nidulans. [Ref: Stanbury, Principles of fermentation

technology]

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FIG. 1.9. Diagrammatic representation of the mitotic division of a eukaryotic cell containing two chromosomes. The nuclear membrane has not been portrayed in the figure. [Ref: Stanbury, Principles of fermentation

technology]

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•Mitotic crossing over involves the exchange of distal segments between chromatids of homologous chromosomes shown in Fig. 1.10.

FIG. 1.10. Diagrammatic representation of mitosis including mitotic crossing over.

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• Haploidization is a process which results in the equal distribution of chromatids between the progeny of a mitosis. Fig. 1.11

FIG. 1.11. Diagrammatic representation of mitosis involving haploidization.

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• Protoplasts are cells devoid of their cell walls and may be prepared by subjecting cells to the action of wall degrading enzymes in isotonic solutions. Protoplasts may regenerate their cell walls and are then capable of growth as normal cells. • Protoplast fusion has been demonstrated in a large number of industrially important organisms including Streptomyces spp. (Hopwood et al., 1977), Bacillus spp. (Fodor and Alfoldi, 1976)

The application of protoplast fusion techniques

[Ref: Stanbury, Principles of fermentation technology]

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The improvement of industrial strains:

Although a strain may produce a very high level of a metabolite it would be unsuitable for a commercial process if its productivity were extremely unstable, or if the organism's oxygen demand were such that it could not be satisfied in the industrial fermenter available for the process.

Therefore, characteristics of the producing organism which affect the process may be critical to its commercial success. Thus, it may be desirable to modify such characteristics of the producing organism which may be achieved by selecting natural and induced variants and recombinants.

[Ref: Stanbury, Principles of fermentation technology]

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Important characteristics for strain improvement• The selection of stable strains• The selection of strains resistant to infection• The selection of non-foaming strains• The Selection of strains which are resistant to components in the medium• The selection of morphologically favourable strains• The selection of strains which are tolerant of low oxygen tension• The elimination of undesirable Products from a production strain• The development of strains producing New fermentation products

[Ref: Stanbury, Principles of fermentation technology]

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Conclusion:

A number of genetic and molecular genetic methods are available to improve fermentation product yields and other strain characteristics. The methods used for the improvement of the strain are effective yet a bit complicated too. The main reason of using such methods is to attain an improved and stable strai that can be used at industrial or commercial level.

•The ability of the producing strain to maintain its high productivity during both culture maintenance and a fermentation is a very important quality.

Example: Woodruff and Johnson (1970) selected a double auxotrophic mutant of Micrococcus glutamicus requiring both homoserine and threonine and compared its lysine-producing properties with those of a homoserine auxotroph.

The selection of stable strains:

[Ref: Stanbury, Principles of fermentation technology]

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•Bacterial fermentations may be affected very seriously by phage infections, which may result in the lysis of the bacteria.

•A possible method for reducing of failure due to phage contamination is to select bacterial strains which are resistant to the phages isolated in the fermentation plant (Hongo et al., 1972)

The selection of strains resistant to infection:

[Ref: Stanbury, Principles of fermentation technology]

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• Foaming during a fermentation may result in the loss of broth cells and product via the air outlet as well as putting the fermentation at risk from contamination.

• Thus, foaming is normally controlled either by the chemical or mechanical means, but this task may be made easier if a non-foaming strain of the commercial organism can be developed.

•The selection of non-foaming strains:

[Ref: Stanbury, Principles of fermentation technology]

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Polya and Nyiri (1966) applied this approach to the isolation of mutants of P. chrysogenum resistant to phenylacetic acid, a precursor of penicillin and toxic to the organism at high concentrations.

The selection of strains which are resistant to components in the medium:

[Ref: Stanbury, Principles of fermentation technology]

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Backus and Stauffer (1955) recognized the influence of the genetic of a strain on the morphology of P. chrysogenum in submerged culture and its role in controlling foaming and broth filtration characteristics

The selection of morphologically favorable strains:

[Ref: Stanbury, Principles of fermentation technology]

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Example, Mindlin and Zaitseva (1966) isolated a lysine-producing strain which maintained its productivity under aeration conditions which decreased the parental strain productivity by almost a half.

The selection of strains which are tolerant of low oxygen tension:

[Ref: Stanbury, Principles of fermentation technology]

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• Athough an industrial micro-organism may produce large quantities of a desirable metabolite it may also produce a large amount of a metabolite which is not required, is toxic or may interfere with the extraction process.

• An example in the penicillin-producing strains is the elimination of the production of the yellow pigment, chrysogenein, selection of non-pigmented mutants which made the extraction of the antibiotic much simpler (Backus Stauffer, 1955).

The elimination of undesirable products from a production strain:

[Ref: Stanbury, Principles of fermentation technology]

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• The isolation of organisms from the natural environment synthesizing commercially useful metabolites an expensive and laborious process. • Therefore, means of producing novel compounds which may be some industrial significance have been attempted.

The development of strains producing new fermentation products:

[Ref: Stanbury, Principles of fermentation technology]

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Conclusion

A number of genetic and molecular genetic methods are available to improve fermentation product yields and other strain characteristics. The methods used for the improvement of the strain are effective yet a bit complicated too. The main reason of using such methods is to attain an improved and stable strain that can be used at industrial or commercial level.

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Reference• Stanbury F. Peter , 2003, Strain Improvement, Principles of fermentation technology, Great Britain by MPG Books Ltd, Bodmin, Cornwall, second Edition, Pg. 43-82. •Tortora J. Gerard, 2010, Strain, Pearson Benjamin Cummings, San Francisco, USA, tenth edition, Pg. no. 18•Prescott M. Lansing, 2007, Strain, The McGraw-Hill Companies, Inc., New York, America, seventh edition, Pg. no. 425•Black G. Jacquelyn, 2008, Strain, JohnWiley & Sons, Inc., pg no. 242

• Net Source: www.cheric.org www.springerlink.com www.jhu.edu www.indianscience.in