Bioreactors are a vessel or device in which biological reaction takes place and they maintain a sustainable environment for cell growth or product formation.
|Schematic presentation of different types of bioreactors used for plant cell suspension culture.
Source: Plant Tissue Culture: Theory and Practice (1983) by S. S. Bhojwani and M. K. Razdan.
Plant cell culture bioreactors are different from microbial fermenters because of the differences in cellular properties of plants and microbes. However, did you know that not all of the cell culture bioreactors are the same?
Yes. Today, various cell culture bioreactors are available and serve different purposes. In the article, “Large-scale tissue culture: Methods and Application,” you can read about the steps to culture plant cells in bioreactors at a larger scale. But, how do you know which bioreactor to choose?
This article will give you a brief description of the different types of bioreactors used to culture plant cells and their applications.
Characteristic features of cell culture bioreactors
Despite some differences in the bioreactors, some characteristics should be followed by all the cell culture bioreactors which are explained below:
- Aeration and low shear mixing
- Adequate dispersion of gas
- Homogenous mixing
- Easy handling
- Long-term stability and sterility
- Ease of scale-up
- Ability to control temperature, pH, and nutrient concentration inside the reactor.
- Avoid all possible contamination of the culture
Types of Bioreactors
Bioreactors are classified by their continuous phase as well as their usability and purpose.
Continuous Phase Types
Depending on the continuous phase of cell suspension cultures, they are divided into three types: Liquid phase Bioreactors, Gas-phase Bioreactors, and Hybrid Bioreactors.
- Liquid phase Bioreactors: In these bioreactors, plant cells are continuously immersed and the air is supplied to the culture by bubbling it through the media. The drawback of these bioreactors is growth inhibition due to the low solubility of gases and insufficient nutrient transfer.
- Gas-phase Bioreactors: These include the spray reactor and the mist reactor. These reactors are helpful in organ cultures, such as hairy roots and they overcome the limitation of oxygen, unlike liquid phase reactors.
- Hybrid Bioreactors: These reactors can switch from the liquid phase to the gas phase operation after the inoculation. They are the combination of submerged and emerged bioreactors. They can give excellent biomass productivity of >1g dry weight/L/day. They include Wilson bioreactor that is used for hairy root culture.
Usability and Purpose Types
Depending on the usability and purpose of the bioreactor The bioreactors can also be defined depending on the functions they perform. So, in this section, a brief of bioreactors is given depending on their usability and purposes.
- Tank Reactors: Tank reactors have mixing stirrers and the mixing blades that are designed in a way to reduce or control the damage to cultured cells. This is important because if cells undergo hydrodynamic stresses, they lose their viability and proliferation capability.
- Air is essential for the growth of the cells, so, in the tank reactors, it is supplied through sparger forming bubbles. If the size of the bubble is much larger or smaller than the cultured cells, it can create hydrodynamic or shear stress on the cell surface. In the tank reactors, sparger forming bubbles and agitating blades are designed to protect the cell from this kind of stress.
|Figure:Schematic diagram of the Stirred Tank Reactor.
- Bubble Beds: Bubble beds or bubble columns transfer mass and heat by sparging large columns with air or gas injected by a static or dynamic gas distributor.
- The major difference between the stirred tank reactors and bubble beds is that the bubble beds are not equipped with the impellers (rotors).
- The liquid mixing is done in bubble motion and draft tubes are used to stabilize the circulating flow.
- Air-lift bioreactors are also included in this category.
|Figure:A schematic diagram of Air-lift Bioreactor
- Rotary Reactors: These reactors are designed to reduce the dynamic impacts due to hydrodynamic stresses. Cells are suspended in a rotating drum and the mixing is done by scoop paddles.
- Oxygen is supplied through the culture medium surface in an adequate quantity to preserve cell viability. This is used for the cell suspension culture (especially for the culture of C.roseus or L. erythrorhizon).
|Figure:A schematic diagram of Rotary Bioreactor.
- Hairy Cell Culture Reactors: These bioreactors are specially designed to collect the secondary compounds from the root growth. Besides the production of secondary, these are also used in studies of transgenic plants.
- They include the mist reactor, radical bioreactor, and Wilson bioreactor, which is specially designed to maintain regular and homogenous liquid flow. Also, the oxygen flow is enough even to the denser region of the roots, which saves the culture from oxygen starvation.
|Figure:A schematic diagram of the Mist Bioreactor.
- Product Separation Bioreactors: The bioreactor’s use in the accumulation of secondary metabolites is well-known. However, the major drawback of the system is the feedback inhibition, due to product secretion in the media. In this case, a system designed for the separation of the accumulated products is most efficient. It includes two steps: extraction and adsorption.
- The extraction of hydrophobic products can be done using organic solvents or foam separation methods. And the best bioreactor of this kind is a membrane bioreactor.
|Figure:A schematic diagram of product separation by floss floatation (foam separation) method.
- Immobilized Cell Culture Reactors: The immobilization of plant cells in the bioreactor is performed by using gels, such as calcium alginate, carrageenan, agarose, and sol-gel materials. Some other methods include attachment of the cells to supporting carriers, such as chitosan or chitin.
- The immobilized cells in gel matrices protect them from stress and show stable metabolic activity. It includes membrane reactors, bubble beds, and packed bed reactors.
|Figure:A schematic diagram of the Membrane Bioreactor.
- Furusaki, S., & Takeda, T. (2017). Bioreactors for Plant Cell Culture. Reference Module in Life Sciences. DOI:10.1016/b978-0-12-809633-8.09076-2 .
- Bhojwani S. S. and Razdan M. K. (1983). Plant Tissue Culture: Theory and Practice. Elsevier publications.