Biomedical Engineering Reference
In-Depth Information
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Low protein-making
plasmid-containing cell
Active protein-making
plasmid-containing cell
plasmid-free cell
FIGURE 16.12 Genetic instability: cells contain active protein-making plasmids, p , must direct their cellular
resources away from growth and hence more-plasmid containing cells grow slower; plasmid-free cells grow the
fastest.
engineered organisms. There is a tension between the goal of maximal target-protein
production and the maintenance of a vigorous culture. The formation of foreign protein
takes away the cellular resources otherwise for natural growth. In addition, the formation
of large amounts of foreign protein is always detrimental to the host cell, often lethal.
Fig. 16.12 shows a schematic of the growth comparison among cells with different active
protein e plasmid levels. Cells that lose the capacity to make the target protein often grow
much more quickly and can displace the original, more productive strain. This leads to
genetic instability: some cells could loose the plasmid, and the genetic differences grow
with time as the plasmid-free cells grows much faster than the genetically engineered cells
with active protein-making plasmid. Genetic instability can occur due to segregational loss,
structural instability, host cell regulatory mutations, and the growth-rate ratio of plasmid-free or
altered cells to plasmid-containing unaltered cells. Genetic instability can occur in any
expression system. Fig. 16.12 illustrates this problem by considering gene expression
from plasmids in bacteria.
Genetic instability is illustrated in Fig. 16.13 for continuous culture. In continuous culture as
demonstrated in Example 16-2, Fig. E16-2.2 , the steady state corresponds to the intercept of the
specific biomass growth rate (intrinsic to cells) and the dilution rate (intrinsic to reactor oper-
ation or culture environment). In the simplistic case of two uninteracting species, the mere
difference in the specific growth rates leads to the elimination of one species as shown in
Fig. 16.13 . The specific growth rate of A is lower than the specific growth rate of species B at
a given (but same) common limiting substrate concentration. When the culture conditions
are fixed, the final (steady-state) substrate concentration is determined by the amount of
biomass grown in the reactor. Since only one substrate concentration can be realized in the
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