Cells derive energy from nutrients to synthesize compounds required for life reproduction and functions. This energy is provided by biochemical reactions that form a biochemical network systems called as metabolic pathways. Molecules that flow through the metabolic pathways are due to the response of environmental stress. These molecules, known as secondary metabolites, are defensive in nature, such as when a microorganism secretes an antibiotic that kills other competing bacterial species. Bioprocess scientists aimed at capturing the value of a cell's metabolic pathways via identification of end products or valuable intermediates. The primary goal was to develop methods that generate these molecules at high concentrations.

Since the 1950s, random mutation of a DNA uses chemical mutagens or radiation to screen survivors to achieve the desired improvements to its metabolic pathways and to generate targeted products. Techniques in molecular biology are developed since 1975; however, the introduction of microbial genome sequencing in 1995 has dramatically changed this approach. Metabolic pathways can be altered by introducing new genes; modifying the existing ones; or blocking gene expression in an organized way. Microarray techniques allow the screening of altered microbes to check for the desired traits.

Operational and Strategic Control

Metabolic pathways have potential operational and strategic controls. Operational control occurs via built-in governors present in the protein activity, which respond to the concentration and presence of the metabolites. These metabolites regulate enzyme activity expressed by microbes. The cell's ability in regulating genes that direct the allotted resources to generate enzymes and proteins that form the metabolic pathway's functional components achieves strategic control.

In strategic genetic control, gene expression control alters the metabolism rate by altering the enzyme catalysts concentration and determining the presence or absence of a particular metabolic pathway, thus affecting the changes very slowly. While, operational control via feedback inhibition is very fast and occurs when the metabolic pathway product or an intermediate compound interacts with a key enzyme. This occurs either early in the metabolic pathway or at critical branch points in such a manner as to slow down or inhibit the reaction rate. This, in turn, slowdowns the molecule flux via the metabolic pathway.


Microorganism that require reduced organic molecules, like amino acids, which cannot be synthesized by them, but are vital for their growth, are known as auxotrophs. The missing amino acids are either supplied as synthetic components or complex media into the culture medium which grows the microorganisms. Growth essential amino acids are given as individual components in the synthetic media. On the other hand, proteins or other sources, such as peptone, steep water, or yeast extract, supply the missing amino acids in a complex media. These amino acids are referred as essential amino acids because they play vital role in good life and health. Humans obtain essential amino acids through diet.

Auxotrophy Process

In the process of auxotrophy, end products and intermediates from microbial metabolism are accumulated by reducing the accumulation of repressive metabolites or inhibitory metabolites inside the cell. Auxotrophs are nutritionally deficient and rely on energy from external sources. Some examples of auxotrophs include Corynebacterium glutamicum and Bacillus subtilis, which produce amino acids in large amounts. Microorganisms are selected, engineered, or controlled to avoid product destruction once feedback inhibition (operational control) and repression (strategic control) are achieved.


There lies various challenges and complexity in directing metabolic pathways to obtain commercially valuable products. This requires the ability to screen millions of microorganisms in quick time, until the desired microorganism with the desired property is identified. Though modern biotechnology deals with the changes done to the genetic makeup of a microbial cell, screening to differentiate cells is still required. Various methods have been successfully employed in identifying the desired cells. Basic microorganism is subjected to chemical mutagens or radiation and undergoes random mutation to identify mutants that offer resistance to feedback repression or inhibition.

With advances in molecular genetics, scientists have now been able to direct enzymatic changes at genetic level. The key objectives of these techniques are to modify a cell's genetic makeup either by adding or deleting or altering gene and their expression. Examples of commercially important products that were developed from microorganisms years ago include penicillin from Penicillium chrysogenum and lysine from Corynebacterium glutamicum. They represent a mature modern biotechnology.

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