Question

Give an example of a perfusion reactor and fed batch reactor, why is it being used, and what are the advantages and disadvantages?

Answer 1

A bioreactor refers to any manufactured device or system that supports a biologically active environment. In one case, a bioreactor is a vessel in which a chemical process is carried out which involves organisms or biochemically active substances derived from such organisms.

Fed-batch culture is, defined as an operational technique in biotechnological processes where one or more nutrients (substrates) are fed (supplied) to the bioreactor during cultivation and in which the product(s) remain in the bioreactor until the end of the run.

Perfusion bioreactor is continually removing cell waste products and media depleted of nutrients by cell metabolism. Fresh media is provided to the cells at the same rate as the spent media is removed. example : chemostat.

Traditional fed-batch bioreactor systems consist of tanks that are usually between 10,000-25,000 liters. Cells are cultured in batches that typically run between 7-21 days by which time media nutrients have been consumed and toxic waste has begun to accumulate. During the run, cells secrete the protein of interest into the media and at the end of the run the protein is separated from cell mass as a batch. Typical product yields are in the range of 1 to 4 grams per liter depending on the clone and antibody. While regular improvements have moved product yield from under 1 gram per liter to where they are now, it has not improved other issues in fed batch manufacturing including large manufacturing footprints and challenges with scalability.

In contrast, perfusion bioreactors culture cells over much longer periods, even months, by continuously feeding the cells with fresh media and removing spent media while keeping cells in culture. In perfusion there are different ways to keep the cells in culture while removing spent media. One way is to keep the cells in the bioreactor by using capillary fibers or membranes, which the cells bind to. Another does not bind the cells, but rather relies on filtration systems that keep the cells in the bioreactor while allowing the media to be removed. Another method is the use of a centrifuge to separate cells and return them to the bioreactor.

fed batch systems

Advantages of fed batch systems:

1 Higher yield, resulting from a well-defined cultivation period during which no cells are added or removed.

2 Increased opportunity for optimizing environmental conditions of the microorganisms in regard to the phase of growth or production and age of the culture.

3 Nearly stationary operation, important with slightly mutating microorganisms and those at risk for contamination.

Disadvantages include:

1 Lower productivity levels due to time for filling, heating, sterilization, cooling, emptying and cleaning the reactor.

2 Higher costs in labour and/or dynamic process control for the process.

perfusion bioreactors

1 offer increased opportunities for system investigation and analysis. As the variables remain unchanged, a benchmark can be determined for the process results, and then the effects of even minor changes to physical or chemical variables can be evaluated. By changing the growth-limiting nutrient, changes in cell composition and metabolic activitycan be tracked. The constancy of the continuous process also provides a more accurate picture of kinetic constants, maintenance energy and true growth yields.

2 provides a higher degree of control than a batch culture. Growth rates can be regulated and maintained for extended periods. By varying the dilution rate, biomass concentration can be controlled. Secondary metabolite production can be sustained simultaneously along with growth. In steady state continuous culture, mixed cultures can be maintained using chemostat cultures – unlike in a batch process where one organism usually outgrows another.

3 Bioreactors operated as chemostats can be used to enhance selectivity for thermophiles, osmotolerant strains or mutant organisms with high growth rates. Also the medium composition can be optimized for biomass and product formation using a pulse- and shiftmethod that injects nutrients directly into the chemostat. As changes are observed, the nutrient is added to the medium supply reservoir and a new steady state is established.

4 Because of the steady state of culture, the results are not only more reliable but also more consistent leading to a better quality product. 5 It also results in higher productivity per unit volume, as time consuming tasks, such as cleaning and sterilization are unnecessary. 6 The ability to automate the process makes it more cost-efficient and less sensitive to the impact of human error.

Disadvantages include:

1 The control of the production of some non-growth related products is not easy. For this reason, the process often requires feed-batch culturing and a continuous nutrient supply.

2 Wall growth and cell aggregation can also cause wash-out or prevent optimum steady-state growth.

3 The original product strain could be lost over time if a faster growing one overtakes it.

4 The viscosity and heterogenous nature of the mixture can also make it difficult to maintain filamentous organisms.

5 Long growth periods not only increase the risk of contamination but also dictate that the bioreactor must be extremely reliable and consistent, incurring a potentially larger initial expenditure in higher quality equipment.

Thank You!