186. A recombinant protein is produced in Escherichia coli by two stage continuous cultivation at steady state.
Upon induction of the culture in stage II, the specific growth rate of the culture decreased considerably. For
an input flow rate in stage I of 100mL/h, a steady rate of product formation can only be obtained
1. when stage I is larger than stage II
2. when stage I is smaller than stage II
3. when both the stages are of the same size
4. for all possible ratios of the sizes of the two stages

Introduction:

Two-stage continuous cultivation is a common approach in bioprocessing, particularly when producing recombinant proteins in microorganisms like Escherichia coli. This method is advantageous because it allows precise control over both the growth phase (stage I) and the production phase (stage II) of the culture. A significant challenge during this process is maintaining a steady rate of product formation, especially when the specific growth rate of the culture decreases upon induction in stage II.

In this article, we explore the relationship between the flow rates, stage sizes, and steady-state product formation during two-stage continuous cultivation and discuss how the relative sizes of stage I and stage II influence the system’s ability to maintain a steady rate of protein production.

Key Concepts in Two-Stage Continuous Cultivation:

1. Stage I and Stage II:

   • Stage I: This is the growth phase where the microorganism is proliferating but not producing the recombinant protein yet. The input flow rate in stage I must be carefully controlled to support optimal growth.

   • Stage II: This is the production phase where recombinant protein is synthesized after induction. A decrease in specific growth rate typically occurs upon induction, as the cells divert their energy from growth to protein production.

2. Specific Growth Rate: The specific growth rate (μ) is a measure of the rate at which the cells multiply in the culture. When the recombinant protein production is induced in stage II, the cells often experience stress, leading to a reduction in their growth rate. The challenge lies in maintaining a high enough growth rate in stage II while ensuring maximal protein production.

3. Steady-State Product Formation: Achieving steady-state product formation requires a delicate balance between the growth rate and the rate of recombinant protein synthesis. Any imbalance can lead to inefficient protein production or unstable culture conditions.

Flow Rate and Stage Size Relationship:

In a two-stage continuous cultivation system, the flow rate from stage I to stage II, as well as the relative sizes of the two stages, play an important role in maintaining steady-state conditions. The key question is whether the steady rate of product formation can only be obtained when certain conditions are met.

1. When Stage I is Larger than Stage II: If stage I is larger than stage II, this may ensure that the cells have a higher volume for growth, which can help support optimal conditions for cell proliferation before entering the production phase. However, if the size difference is too large, the cells may not have enough time or nutrients in stage II to support steady protein production.

2. When Stage I is Smaller than Stage II: If stage I is smaller than stage II, the cells will have a smaller growth volume, but a larger volume for protein production. This setup could be useful when product formation requires a longer residence time in stage II to maximize the yield. However, an excessively large stage II may lead to a dilution effect that impacts steady-state product formation.

3. When Both Stages Are of the Same Size: If the stages are of equal size, the system could achieve a balance between growth and production, assuming the flow rate and induction conditions are optimized. This setup could provide a stable environment for both cell growth and product formation.

4. For All Possible Ratios of Stage Sizes: While various ratios of stage sizes can be tested, not all ratios will guarantee steady-state product formation. Some ratios may result in inefficient growth or insufficient time for protein production, leading to instability in the process.

Conclusion:

In order to achieve a steady rate of recombinant protein production in a two-stage continuous cultivation system, careful consideration must be given to the size of each stage. A key insight is that steady-state product formation is most likely achieved when stage I is larger than stage II. This ensures that there is enough room for optimal cell growth before the induction phase, where the cells shift towards protein production, despite the reduced growth rate.

Answer:

The correct answer is 1. when stage I is larger than stage II.