Continuous culture is a process in which nutrients are continuously added to the bioreactor while the culture broth (containing cells and metabolites) is simultaneously removed.
The culture broth volume remains constant due to a balanced feed-in and feed-out rate, ensuring that consumed nutrients are replenished and toxic metabolites are eliminated.
Continuous culture simplifies scale-up by maintaining a constant working volume, allowing the use of a constant power-to-volume strategy.
It allows for the establishment of optimum conditions for maximum and prolonged product synthesis.
It enables stable product quality due to a steady-state condition, where the culture remains homogeneous with constant biomass and metabolite concentrations.
Higher productivity per unit volume is achieved, as time-consuming processes like cleaning and sterilization are minimized.
Steady-state cultures can be maintained for extended periods—days, weeks, or even months—significantly reducing downtime and enhancing economic competitiveness.
Principle of Continuous Culture
A continuous flow system includes a reactor where reactants are steadily pumped in and products are continuously removed.
The operation of the system is influenced by how material flows through the reactor, which is determined by the reactor’s design, and by the kinetics of the reaction occurring inside.
In continuous culture, growth-limiting nutrients can be maintained at steady-state concentrations, allowing microorganisms to grow at submaximal rates.
During steady-state conditions, both the cellular growth rate and environmental parameters such as metabolite concentrations remain constant.
Continuous culture enables easy monitoring and control of parameters like pH, oxygen tension, concentration of excretion products, and population densities.
Process of Continuous Culture
In continuous culture, an open system is maintained where one or more feed streams with essential nutrients are continuously introduced, and an effluent stream containing cells, products, and residuals is continuously removed.
A steady-state is achieved by keeping the volumetric flow rates of feed and effluent streams equal.
The culture volume remains constant, and all nutrient concentrations are maintained at steady-state levels.
This process prolongs the exponential growth phase and minimizes the formation of byproducts.
Continuous fermentation is monitored based on microbial growth activity or by-product formation using the following methods:
A. Turbidostat method
Cell growth remains constant while the flow rate of fresh media varies.
Cell density is regulated according to a set turbidity value generated by the growing cell population, with continuous supply of fresh media.
B. Chemostat method
Nutrients are supplied at a constant flow rate, and cell density is regulated based on the availability of essential nutrients.
The growth rate is controlled by adjusting substrate concentrations such as carbon, nitrogen, and phosphorus.
Chemostat continuous culture characteristics:
Medium and cells are continuously exchanged.
Cell density remains constant.
Growth is steady-state.
The system is open.
C. Plug-flow reactor
Culture solution moves through a tubular reactor without back mixing.
Nutrients enter in discrete “plugs” that flow axially through the reactor.
The medium flows steadily through the tube, and cells are recycled from the outlet back to the inlet.
Applications of Continuous Culture
Continuous culture fermentation is applied in the production of single-cell proteins, organic solvents, and starter cultures.
It is utilized in the manufacturing of products like beer, fodder yeast, vinegar, and baker’s yeast.
It plays a significant role in the industrial production of secondary metabolites, such as antibiotics produced by Penicillium or Streptomyces species.
Continuous culture has been evaluated for its effectiveness with Corynebacterium glutamicum mutant B-6 in producing L-lysine.
It is also employed in municipal wastewater treatment processes.
Limitations of Continuous Culture
Maintaining sterility over long-term cultivation can be difficult, and downstream processing may present significant challenges.
Controlling the production of non-growth-related products is often complex, necessitating the use of fed-batch culturing alongside a continuous nutrient supply.
The viscous and heterogeneous nature of the culture mixture can make it difficult to maintain filamentous organisms.
There is a risk that a faster-growing strain may dominate the culture, leading to the eventual loss of the original product-producing strain.
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