Bioreactors cell growth

The Monod equation is not the only unstructured, nonsegregated empirical model of bioreactor cell growth. Others can be found in the summaries provided by Bellgardt (2000a), Dunn et al. (2003), or Nielsen et al. (2003). 

A special CSTR fermentation known as a chemostat bioreactor is used for microbial cell growth. The rate of biomass generation is given by ... 

The influence of mechanical forces on cell viability is of great importance when growing the cells in agitated systems. By far the greatest amount of work reported in the literature has been done on suspension cells but adherent cells also experience shear forces not only in bioreactors also in vivo. Therefore, most research has be done on endothelial cells but studies exists done on non-endothelial cells. The influence of shear forces on cell growth, morphology and productivity will be discussed as well as possibilities of making the cells more resistant. 

In this paper the fundamental aspects of process development for the production of core and virus-like particles with baculovirus infected insect cells are reviewed. The issues addressed include particle formation and monomer composition, chemical and physical conditions for optimal cell growth, baculovirus replication and product expression, multiplicity of infection strategy, and scale-up of the process. Study of the differences in the metabolic requirements of infected and non-infected cells is necessary for high cell density processes. In the bioreactor, the specific oxygen uptake rate (OURsp) plays a central role in process scale-up, leading to the specification of the bioreactor operational parameters. Shear stress can also be an important variable for bioreactor operation due to its influence on cell growth and product expression. 

In this part the determination of critical variables for bioreactor operation and scale-up are reviewed. Special focus is put on the specific oxygen uptake rate (OURsp) and its influence upon the specification of bioreactor operational parameters. Shear stress can also be an important variable for process design and its importance in cell growth and product expression is discussed. 

Temperature Ihe temperature in a bioreactor is an important parameter in any bioprocess, because all microorganisms and enzymes have an optimal temperature at which they function most efficiently. For example, optimal temperature for cell growth is 37 °C for Escherichia coli and 30 °C for Saccharomyces sp, respectively. Although there are many types of devices for temperature measurements, metal-resistance thermometers or thermistor thermometers are used most often for bioprocess instrumentation. The data of temperature is sufficiently reliable and mainly used for the temperature control of bioreactors and for the estimation of the heat generation in a large-scale aerobic fermentor such as in yeast production or in industrial beer fermentation. 

Cells often take a time to adjust to the bioreactor and a small initial drop in viability is normal. If the cells are not growing and the viability has dropped below 50%, and contamination has been ruled out, there are several options to try 1) re-seed the bioreactor at a higher cell density 2) adjust the rolling speed 3) increase the serum concentration in the nutrient module 4) add OPI at 1/50 to promote cell growth. 

The main advantages of continuous cell lines are (i) faster cell growth, achieving high cell densities in culture, particularly in bioreactors (ii) the possible use of defined culture media available in the market, mainly serum-free and protein-free media and (iii) the potential to be cultured in suspension, in large-scale bioreactors. 

Stirred-tank bioreactors Air-lift bioreactors Wave bioreactors Microcarrier-based systems Packed-bed bioreactors Fluidized-bed bioreactors Hollow-fiber bioreactors Bioreactors providing surfaces for attached cell growth (roller bottles, CellCube , Cell Factory)... 

Bioreactors providing surfaces for attached cell growth... 

The low volumetric productivities that characterize batch cultivation processes are a disadvantage for the use of this operation mode for production. However, a variant known as repeated batches is an interesting alternative. It consists of initially carrying out a batch cultivation for the time needed to attain the desired product concentration. At that moment, just a part of the bioreactor contents is harvested. The remaining cell suspension inside the bioreactor is then used as inoculum for a new batch, by filling the vessel with fresh medium. This procedure can be repeated several times, until a decrease in cell growth or product formation is observed. The use of repeated batches allows a decrease of the time the bioreactor is non-productive. This eliminates the time periods that would be necessary for cleaning and sterilization between each batch. 

A limiting condition when carrying out continuous culture is the situation known as bioreactor wash-out. This condition is characterized by a cell removal rate at the bioreactor outlet that is equal to or higher than the cell growth rate, such as that shown in Figure 9.16B after D has been increased to 0.8 d 1. The wash-out condition occurs when the dilution rate is increased to a threshold value called Dwash-out which can be derived from Equation 7, considering steady state and adopting Pmax for p. 

If a separator, used as a cell retention device in a perfusion bioreactor, is operating with E < 100%, some of the cells are lost in the perfusate. In such a situation, the apparent specific cell growth rate p p is given by Equation (19). 

Consequently, the maximum perfusion rate possible that can be used in a perfusion bioreactor is a function of both the specific cell growth rate and the cell retention efficiency. 

As mentioned in Chapter 9, since production scale-up is related to the increase of cell culture surface for adherent cells, consideration must be given to the relationship between the surface area available for cell growth and the bioreactor volume (Kent and Mutharasani, 1992). 

Maranga L, Cunha A, Clemente J, Cruz PE, Carrondo MJT (2004), Scale-up of viruslike particles production effects of sparging, agitation and bioreactor scale on cell growth, infection kinetics and productivity, J. Biotechnol. 107 55-64. 

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