Cell culture bioreactor operation

The development of novel bioreactors and bioreactor operating strategies has the potential to improve the performance of cell culture systems. Some progress has been made in this area to enhance foreign protein production in plant tissue culture. 

The bioreactor operation mode is normally defined at the outset of process configuration. Insect cells have been cultured in almost all known cultivation modes batch [10], repeated-batch [70], perfusion [71-74], fed-batch [75, 76], semi-continuous [77,78] and continuous [79]. In spite of this multitude of different strategies, the batch or, eventually, fed-batch mode is normally preferred due to the lytic infection cycle of the baculovirus. 

Commercial manufacturing operations in biotechnology usually employ bioreactors or fermentors for product expression. In this discussion, the term fermentor will refer to bacterial or fungal processes and the term bioreactor to animal cell cultures. While extensive description of the operation... 

Unit operations for biological products obtained from fermentation or cell culture can largely be subdivided into four parts medium preparation, inoculums expansion, bioreactor, and harvest operations. 

There are a variety of types of bioreactors described in the literature. Among them, the stirred tank bioreactor is the most commonly employed due to its record of performance and ease of operation. Cells growing in bioreactors take up nutrients from the culture medium and release products, byproducts, and waste... 

Several different bioreactor configurations have been described for use in cell culture and fermentation applications. These include stirred tanks, airlift, and hoUow-fiber systems. The majority of bioreactor systems in use for cell culture applications are still of the stirred-tank type. These systems have been used for batch, fed-batch, and perfusion operations. It would not be possible to adequately cover the field of stirred-tank scale-up in the space available here. Instead, this section will touch briefly on the important issues in bioreactor scale-up. For detailed methodologies on stirred-tank bioreactor scale-up, the reader is referred to several review papers on the topic [20,27,28]. 

Fluidized-bed bioreactors are also used mainly for the culture of cells attached to microcarriers. Flowever, in this case, microcarriers with a higher density should be employed. In these bioreactors, a given amount of microcarriers is fluidized inside a compartment of the bioreactor, avoiding cell damage due to bubble explosion. Figure 9.10 (see color section) shows a photograph and an operational scheme of this type of bioreactor. 

In this operation mode, it is possible to mitigate the major limitation of continuous cultures, that is, the low productivity due to the loss of cells in the bioreactor outlet. In perfusion, this issue is overcome by using a cell retention device to maintain cells inside the bioreactor. Figure 9.17 shows a scheme of a stirred-tank bioreactor operating in perfusion mode, as well as the kinetic behavior of a perfusion run. 

The operation of cultures in perfusion mode is possible for almost all existing bioreactor types. Heterogeneous bioreactors are usually operated in perfusion mode, and homogeneous bioreactors can be if a solid-liquid separation device (cell retention device) is used (see Chapter 11). 

Such information about shear effects in plant cell suspension cultures as given above is useful for bioreactor design and operation as well as the optimization of bioreactor environments for plant cell cultures. It may also be beneficial to bioreactor scale-up and high-density cultivation of plant cells for production of useful plant-specific chemicals (pharmaceuticals). In addition, because different cell suspensions can show different degrees of cell sensitivity to shear stress, and shear affects culture viability, cell lysis, and even metabolite secretion, as demonstrated in various cases like cell cultures of tobacco, Catharanthus roseus and Perilla frutescens [17, 54, 61], detailed studies are required for individual cases. 

Various adsorbents have been examined for their potential to increase in situ product separation in plant cell culture. Suspended solid adsorbents were popular, and the use of immobilized adsorbent has been investigated recently [17-20]. The advantages of immobilized adsorbent are that it is easy to use in a bioreactor operation and that it allows adsorbents to be easily separated from culture broth for the repeated use of cells and adsorbents [21, 22]. The design and optimization of in situ separation process for phytochemicals using immobilized adsorbent required a detailed mathematical model. It was difficult to achieve an optimal design based on purely empirical correlations, because the effects of various design parameters and process variables were coupled. 

Alternatively, some subunit viral vaccines can be generated by rDNA techniques and expressed in a continuous cell line or insect cells. Recent advances in bio reactor design and operation have improved the successful production of IPV in large-scale bioreactors. However, roller bottles or flasks are still used for most current vaccine production. Development of insect cell culture will allow for very large-scale liquid suspension culture (143). Several vaccine candidates such as gpl60 for HIV and gD protein for herpes have been demonstrated in the insect cell culture system. However, no vaccine has... 

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