bioreactor studies of heterologous protein production by ...

the daughter ce&. Two models were developed based on the two rnechaaisms for plasmid
replication. In the b t modei, the œlls replicated plasmids such that the total numbu of
plasrnids was the same for aii dividing tells in the population. In the second. the ceUs
produced plasmids at the same rate, inespective of the initial copy number. The resuits
obtained h m those models were compared with the comsponding results hm a
nonstructured model, and pronounced dineremes were found at low growth rates. These
models were able to relate obseniable bulk properties of growth to lrinetic events at the
cell level. ln later work, a general single-celi rnodel for plasmid propagation in
recombinant yeast was developed (Wictnip and Bailey, 1988). This mode1 inciuded the
plasmid burden effects on host ceIl growth rate, and it could be combined with bio~actor
performance equations to simulate a production proces.
A macroscopic population dynamics model for plasmid stabiüty h continuous
culture of recombinant S. cerevisiae with selection pressure was described by Sardonhi
and DiBiasio (1987). Several assurnptions were made, including: (1) planaid-free ceU
could propagate to some degret in the selective media, which could be explained by a
metabolite king excreted into the media by the plasmid£arrying strain. This metabolite
supporkd the growth of plasmid-free œb, (2) plasmid loss occurs at constant pmbabiky;
(3) the growth rate of plasmid£arrying ails is limitai by a singie subsaate described by a
Monod equation; (4) growth rate of the plasmici-free ce& is limiteci by this substcate as
weU as an additional metaboüb descriid using a duaï Monod form; (5) continuous
culture with a constant reactor volume and sterile feed. The Sardonini and DiBiasio