Questions: Industrial Fermentation and Production Microbiology

Question 1

A pharmaceutical company producing recombinant insulin in E. coli needs to maximize protein yield while avoiding substrate inhibition. Which fermentation mode is most appropriate?

• A. Batch fermentation, because its simplicity minimizes contamination risk and operational overhead
• B. Continuous fermentation, because it maintains cells in exponential growth and maximizes volumetric productivity
• C. Fed-batch fermentation, because nutrients can be added at controlled rates to avoid inhibition while extending the productive phase
• D. Batch fermentation, because recombinant proteins are only expressed after all nutrients are exhausted

Answer: (C)

Fed-batch is the industry standard for recombinant protein production. Adding nutrients incrementally prevents substrate inhibition (where high glucose concentrations suppress growth or redirect metabolism) and extends the productive phase beyond what a single batch allows. Continuous fermentation maximizes productivity but requires extraordinary sterility control and is rarely used for complex biologics. Option D describes a common misconception about recombinant expression; induction timing is controlled by inducers (like IPTG), not nutrient depletion.

Question 2

In a stirred-tank bioreactor, why is excessive agitation speed harmful when producing monoclonal antibodies using mammalian cell culture?

• A. Rapid mixing distributes oxygen unevenly, creating anoxic zones that stress the cells
• B. High shear forces generated by the impeller physically damage the fragile membranes of mammalian cells
• C. Excessive agitation raises temperature beyond the optimal range for mammalian cell growth
• D. The impeller consumes ATP that cells would otherwise use for antibody synthesis

Answer: (B)

Mammalian cells lack the rigid cell wall that protects bacteria and yeast. High impeller speeds generate turbulent shear forces that can rupture or stress cell membranes, reducing viability and productivity. This is a fundamental constraint distinguishing mammalian cell culture from bacterial fermentation: the same vigorous agitation that helps E. coli absorb oxygen efficiently would kill CHO cells used for antibody production. Bioreactor design for mammalian cells uses gentler mixing (wave bioreactors, low-shear impellers) to balance oxygen transfer and cell viability.

Question 3

Metabolic engineering improves production yields solely by adding new biosynthetic genes; gene knockouts are not used because removing pathways might harm the organism.

• T. True
• F. False

Answer: False

Knocking out competing pathways is often as important as adding new genes. When carbon flux can be diverted to waste products or growth rather than the desired compound, deleting the enzymes responsible for those competing reactions redirects resources toward the target product. For example, engineering E. coli to produce a desired organic acid may require knocking out pathways that would otherwise consume that acid. The metabolic engineer treats the cell's biochemistry as a network and edits it to maximize flux toward the desired output — deletions and additions are both standard tools.

Question 4

As bioreactor volume scales from laboratory (1L) to industrial (10,000L) scale, maintaining adequate oxygen transfer and mixing uniformity becomes increasingly difficult and requires careful engineering solutions.

• T. True
• F. False

Answer: True

Volume scales as the cube of linear dimensions while surface area scales as the square, so the surface-area-to-volume ratio shrinks dramatically at large scale. Oxygen transfer from spargers to the bulk culture, and heat removal through reactor walls, both become limiting. At the same time, achieving uniform mixing without excessive shear is harder in large vessels. Industrial bioprocessing addresses this through multiple impeller stages, optimized sparger designs, and sophisticated process control — challenges that simply don't arise in a shaker flask.

Question 5

Why would metabolic engineers delete genes from a production microorganism rather than only adding new biosynthetic genes?

Model answer: Competing metabolic pathways divert carbon, energy, and cofactors away from the desired product. By deleting the enzymes responsible for those competing reactions, engineers redirect cellular resources toward the target pathway. Additionally, product-degrading enzymes — if present — would consume the compound as fast as it is made; knocking these out is essential for accumulating yield.

The cell is an interconnected metabolic network, and adding a new pathway doesn't guarantee flux will flow through it if easier routes exist. Classic examples include deleting acetate-producing pathways in E. coli (which waste carbon at high glucose concentrations) or deleting the enzyme that degrades the target compound. Metabolic engineering is fundamentally about reshaping the flux distribution of the entire network, which requires both additions and deletions.