Commercial production suspension cells

For initial studies, plant cell suspensions are typically cultivated in shake flasks agitated by a gyratory shaker, but for the economical production of commercial products, plant suspension cultures must be cultivated in larger bioreactors. 

Radlett PJ, Pay TWF Garland AIM (1985) The use of BHK suspension cells for the commercial production of foot and mouth disease vaccines over a twenty year period. Developments in Biological Standardization 60 163-170. 

Typical commercial cell culture systems include batch or fed-batch suspension reactors and perfused immobilized-cell reactors. However, the transient nature of batch culture causes difficulties in studying the effects of external stimuli on growth, metabolism and product formation. Due to metabolite concentration gradients, and the difficulty of obtaining representative cell samples, immobilized-cell reactors are also poorly suited for the analysis of cell growth and metabolism. As a result it is desirable to use well-defined model systems. Continuous-flow suspension reactors allow metabolic parameters to be measured at steady state, after cells have adapted to new (and possibly inhibitory) conditions. Perfusion reactors (with cells immobilized on suspended or stationary supports) extend these benefits to anchorage-dependent cells, and provide model systems for cell responses in vivo. However, while it is instructive to study the behaviour of cells under well-defined conditions, the results obtained must be verified in the culture system selected for commercial production. 

Diosgenin (Fig. 4) is a steroid sapogenin produced by Dioscorea species. This natural product is used as a precursor for the commercial synthesis of medicinal steroids such as prednisolone, dexamethasone, norethisterone, and metenolone, among others. Microtuber-derived cell suspensions of Dioscorea doryophora were able to accumulate diosgenin at concentrations as high as 3.5% g dw. Embryogenic callus of Tribulus terrestris were also able to accumulate diosgenin (170.7 1.0 pg/g dw). ... 

Commercial scale cultivation of mammalian cells is accompHshed using different technologies roller bottles, microcarriers, suspension (batch, fed-batch or perfusion mode) and hollow fiber bioreactors (Table 2). However, especially for products needed in large amounts, suspension cultivation seems to be the most effective system [4, 5]. Suspension-based systems are characterized by a homogeneous concentration of cells, nutrients, metabolites and product, thereby facilitating scale-up [6] and enabling an accurate monitoring and control of the culture [7]. 

What can be done to achieve this necessary improvement Most plant cell products are secondary metabolites, which means that plant suspension culture systems involve a growth phase and a production phase. In any commercial system, one will probably want to separate physically these two phases. Unfortunately, one has to add a growth hormone such as 2-4-D for a successful growth phase, and these hormones must be completely removed before elicitors are added to the production phase to induce the secondary product. Optimizing this two-stage system is an engineering challenge. 

In spite of these problems, one product of plant cell suspension culture has achieved commercialization. This product, shikonin, is a red dye, produced in Japan at the level of several tons per year. Published reports suggest the optimum yield occurs at a relatively low volumetric mass transfer for oxygen, roughly 10 hr-1 (see Fig. 1). When higher oxygen transfer rates are used, the production rate drops. 

The greatest amount of work on scale-up of bioprocesses relevant to plants has been with cell suspension cultures. The potential for chemicals production and for biomass generation, from large scale plant suspension cultures has indeed been recognized for some time (8-10). Many factors remain to be resolved, however, as evidenced by the fact that only one process during the past 25 years has reached commercialization (5). Some of the barriers to successful commercialization are associated with ... 

Suspensions or dispersions of particles in a liquid medium are ubiquitous. Blood, paint, ink, and cement are examples that hint at the diversity and technological importance of suspensions. Suspensions include drilling muds, foodstuffs, pharmaceuticals, ointments and cremes, and abrasive cleansers and are precursors of many manufactured goods, such as composites and ceramics. Control of the structure and flow properties of such suspensions is often vital to the commercial success of the product or of its manufacture. For example, in consumer products, such as toothpaste, the rheology of the suspension can often determine consumer satisfaction. In ceramic processing, dense suspensions are sometimes molded (Lange 1989) and then dried and sintered or fired into optical components, porcelin insulators, turbine blades, fuel cells, and bricks (Rice 1990 Simon 1993). Crucial to the success of the processing is the ability to transform a liquid, moldable suspension into a solid-like one that retains its shape when removed from the mold. These examples could be multiplied many times over. 

The osmolarity of the cell suspension increases during a fermentation process as a result of metabolic events (0yaas, 1989). The influence on the cells and the product is still unclear. Therefore, with each development of a new cell culture process, a study on the importance of this parameter must be performed. Unfortunately, there is no on-line measurement system commercially available, although sterile installation of a membrane osmometer should be possible. 

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