Biotechnology & Genetic Engineering: Chapter 7 — Microbial biotechnology and fermentation

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7.1 What the microbe is asked to make #

7.1.1 Primary and secondary metabolites #

Primary metabolites — amino acids, nucleotides, central organic acids, ethanol — are part of growth and are normally regulated against surplus. Secondary metabolites, including many antibiotics, are not required for laboratory growth and classically appear as growth slows: trophophase, then idiophase. The split is a useful schedule and a simplification. Microbiology owns the regulation. Manufacture inherits the consequence that forcing growth can lower a secondary titre.

The same organism can be asked for both. Yeast biomass and ethanol are primary; an engineered terpene may behave like neither. Classification follows the pathway’s link to growth, not the price of the product. Carbon spent on product is carbon not spent on cells, so biomass and titre pull apart even for a primary metabolite.

7.2 How a culture is run #

7.2.1 Batch, fed-batch, and continuous #

Fed-batch is how bakers’ yeast avoids the Crabtree effect, ethanol from excess glucose even in air, and how recombinant E. coli reaches high density before a strong promoter is induced. In a chemostat the pump sets growth rate. Above μmax the culture washes out. A mutant that grows faster and makes less product wins a continuous culture and is thrown away at the end of a batch. Unless product formation is growth, continuous operation selects against the production strain.

7.2.2 Aeration and sterility as constraints #

Air-saturated broth holds on the order of 8 mg of oxygen per litre, less when warm or salty, and a dense culture can use that in seconds. The limit is transfer from gas to liquid, not the dissolved stock. Heat, foam, pH, and shear are the same class of constraint. They are not operating instructions.

A rich culture held for days rewards one faster contaminant. Antibiotic selection is a poor production plan: cost, residue, and resistance. Finished genes are usually on the chromosome. How a vessel is sterilised is not taught here. A tank that must keep the organism in is a different problem from a field that must let it grow.

7.3 Better strains #

7.3.1 Selection and engineering stacked together #

Mutagenesis and titre screens raised penicillin from around a few milligrams per litre in early cultures to tens of grams per litre in industrial strains, and left a scarred genome. Analogue resistance can enrich a feedback-blind biosynthetic enzyme, or merely a transport mutant; the product assay decides. Engineering then deletes a competitor, deregulates an enzyme, or adds a pathway, usually in a classically improved chassis. Burden selects for loss of the new genes, so integration beats a plasmid once antibiotics cannot be fed to the tank.

Sequencing a classical production strain lists the scars. It does not turn the original screen into a design. The useful order is often the historical one: select a chassis that already grows under process stress, write a defined change into it, and sequence to confirm that the change you wrote is the one you have.

7.4 Products #

7.4.1 Amino acids, organic acids, and antibiotics #

Corynebacterium glutamicum excretes glutamate when the membrane is stressed, classically by biotin limitation, and makes lysine when aspartokinase no longer obeys lysine. Aspergillus niger excretes citrate under trace-metal limitations that unbalance the cycle. Lactic and acetic acids are the other old overflows. Recovery, not the pathway drawing, often dominates the cost at that scale.

Penicillin from Penicillium and streptomycin from Streptomyces griseus are secondary metabolites. Semi-synthetic drugs are chemical changes to a fermented core; the factory still depends on the fermentation for the scaffold. A higher titre does not restore a drug against a resistant pathogen. Microbiology owns action and resistance. These notes do not describe making or modifying an antibiotic.

7.4.2 Recombinant insulin and industrial enzymes #

Animal insulin came from pancreases. Porcine hormone differs from human at one residue, bovine at three, and some patients made antibodies. Recombinant human insulin was licensed in 1982. Separate A and B chains join chemically in several disulphide arrangements, so yield of the correct pairing is poor. Proinsulin folds with the C-peptide in place, and the peptide is then removed, as in a β cell. Supply became a fermenter. Analogues with altered timing are later protein engineering.

Subtilisin, amylases, glucoamylase, and glucose isomerase are process catalysts. T. aquaticus polymerase works near 72°C because the organism lives hot; a thermostable amylase survives starch cooking; an alkaliphilic protease starts nearer detergent pH. The niche suggests a stability. It does not guarantee expression yield or performance in broth.

7.4.3 Bioremediation in outline #

Some organics are biodegraded if the pathway and the electron acceptor are both present. Hydrocarbons often need oxygen. Some chlorinated solvents are attacked, if at all, only anaerobically. Added laboratory strains often lose to the native community or never meet the pollutant. Metals are not degraded; they can be precipitated or reduced, and the atom remains. Plants that take metals up meet the same limit. Site treatment is environmental engineering and is not given here.

7.5 Key terms, summary, and self-check #

Key terms. Primary metabolite; secondary metabolite; idiophase; batch; fed-batch; chemostat; dilution rate; washout; Crabtree effect; oxygen transfer; analogue selection; proinsulin; extremophile enzyme; bioremediation.

Summary. Primary products are growth-linked overflows. Secondary products often fall if growth is forced. Fed-batch limits a nutrient; a chemostat sets μ equal to D and rewards faster mutants. Oxygen transfer and sterility constrain aerobic factories. Classical screens and defined engineering are stacked. Glutamate, citrate, penicillin, recombinant human insulin, and process enzymes are the product types. An extremophile enzyme is a head start, not a finished process. Bioremediation degrades some organics, moves metals without destroying them, or fails.

Self-check.

1. Why can faster growth lower a secondary titre?

2. What does D set in a chemostat, and what is washout?

3. Why is a few milligrams of dissolved oxygen per litre not the oxygen supply of a long run?

4. Why did proinsulin fold better than separate chains, and why did the human sequence matter?

5. What does a hot-spring niche suggest about an enzyme, and what does it not prove?

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