Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 818

Net specific growth rate of B

Net specific growth rate of A

Continuous culture limit: dilution rate

Common limiting substrate concentration

FIGURE 16.13 Genetic instability: difference in growth rate cause the cell type distribution change in contin-
uous culture.

reactor, both species A and B cannot exist the same time in the continuous culture as one
requires to maintain a higher substrate concentration (slow growth species A) while the other
maintaining a lower substrate concentration (faster growth species B). At the right level of
substrate concentration for faster growth species B, slower growth species A is “starved” (or
grows slower than dilution rate). As a result, the faster growth species remains in the contin-
uous culture.

16.5.1. Segregational Instability
Segregational loss occurs when a cell divides such that one of the daughter cells
receives no plasmids, which is rare, but could occur as illustrated in Fig. 16.14 .Ifthe
distribution of plasmids at division is random, the probability of forming a plasmid-
free cell is given by

P ¼ 2 1P R (16.38)

where P R is the number of plasmid replicative units. Plasmids can be described as high-
copy-number plasmids (P R > 20 copies per cell) and low-copy-number plasmids (sometimes as

low as one or two copies per cell). Low-copy-number plasmids usually have specific mech-
anisms to ensure their equal distribution among daughter cells. High-copy-number plasmids
are usually distributed randomly (or nearly randomly) among daughter cells following
a binomial distribution. For high-copy numbers, almost all the daughter cells receive some
plasmids, but even if the possibility of forming a plasmid-free cell is low (one per million
cell divisions), a large reactor contains so many cells that some plasmid-free cells will be
present (e.g. 1000 l with 10 12 cells/l yields 10 15 cells and 10 9 plasmid-free cells being formed
every cell generation).
The segregational loss of plasmid can be influenced by many environmental factors, such
as dissolved oxygen, temperature, medium composition, and dilution rate in a chemostat.
Many plasmids will also form multimers, which are multiple copies of the same plasmid