Media Components - Vitamins, Growth factors & Metabolic Regulators

Vitamins and Growth Factors

 Many bacteria can synthesize all necessary vitamins from basic elements.  For other bacteria, filamentous fungi and yeasts, they must be added as supplements to the fermentation medium.  Most natural carbon and nitrogen sources also contain at least some of the required vitamins as minor contaminants.  Other necessary growth factors, amino acids, nucleotides, fatty acids and sterols are added either in pure form or for economic reasons, as less expensive plant and animal extracts. 

 Precursors

Precursors are “substances added prior to or simultaneously with the fermentation which are incorporated without any major change into the molecule of the fermentation product and which generally serve to increase the yield or improve the quality of the product”. They are required in certain industrial fermentations and are provided through crude nutritive constituents, e.g., corn steep liquor or by direct addition of pure compounds.  Some fermentations must be supplemented with specific precursors, such as, for secondary metabolite production.  They are often added in controlled quantities and in a relatively pure form, examples include, D-threonine is used as a precursor in L-isoleucine production by Serratia marcesans, and anthranillic acid additions are made to fermentations of the yeast Hansenula anomola during L-tryptophan production.  The use of corn steep liquor as side-chain precursors in penicillin fermentations results in six different penicillins as opposed to the use of phenylacetic acid which results in mainly Penicillin G formation.   

Inducers and Elicitors

If product formation is dependent upon the presence of a specific inducer compound or a structural analogue, it must be incorporated into the culture medium or added at a specific point during the fermentation.  The majority of enzymes of industrial interest are inducible.  

Inducers are often substrates such as starches or dextrins for amylase.   

In plant cell culture the production of secondary metabolites, such as flavanoids and terpenoids can be triggered by adding elicitors.  These may be isolated from various microorganisms, particularly plant pathogens.  

Inducers are often necessary in fermentations of genetically modified microorganisms.  This is because the growth of genetically modified microorganisms (GMMs) can be impaired when the cloned genes are “switched on”, due to the very high levels of their transcription and translation.  Consequently, inducible systems for the cloned genes are incorporated that allow initial maximization of growth to establish high biomass density, whereupon the cloned gene can then be “switched on” by the addition of the specific chemical inducer, e.g., a commercial system developed based on the alcA promoter in Aspergillus nidulans to express human interferon α2.  The system is induced by volatile chemicals like ethylmethylketone which are added when the biomass has increased to an adequate level.

 Inhibitors

Inhibitors are used to redirect metabolism towards the target product and reduce formation of other metabolic intermediates; others halt a pathway at a certain point to prevent further metabolism of the target product.  An example of an inhibitor specifically employed to redirect metabolism is sodium bisulphite, which is used in the production of glycerol by S. cerevisiae.  Some GMMs contain plasmids bearing an antibiotic resistance gene, as well as the heterologous gene(s).  The incorporation of this antibiotic into the medium used for the production of the heterologous product selectively inhibits any plasmid-free cells that may arise. 

Cell Permeability Modifiers

These compounds increase cell permeability by modifying cell walls and/or membranes, promoting the release of intracellular products into the fermentation medium.  Compounds used for this purpose include penicillins and surfactants.  They are frequently added to amino acid fermentations, including processes for producing L-glutamic acid using members of the genera Corynebacterium and Brevibacterium.

 References

1. Industrial Microbiology: An Introduction. Michael J. Waites, Neil L. Morgan, John S
2. Principles of Fermentation Technology- Peter Stanbury, Allan Whitaker, Stephen Hall