bioreactor studies of heterologous protein production by ...

concentration couid Iead to coexistence of plasmid-containing and plasmid-fk celis m
continuous culaire. In an experimental study with E. coli K21 strain being pwn in
nonselective continuous culture, Weber and San (1988) observed that when the dilution
rates were kept constant, the fraction of plasmid-beYing cells decnaîed after a penod of
ame. EventuaUy, the plasmid-fiee ceb displaced the recombinants. However, when the
cek were exposed to forced oscillations m the dilution rate, the reactor culture was able
to maintain a mured population of plasmid-free and plasmid-containing cells for longer
periods. For recombinant yeast, Camt et aL (1989, 1990) showed that cycling of
dissolved oxygen with a frequency of a few minutes could enhance plasmid stability.
Whereas Impoolsup et al. (1989) observed that cyciic variations in dilution rate had a
stabiIizing effect on the 2 pn based yeast plasmid. Stabiluing effkcts of growth rate
(dilution rate) cycling may be simulated in fed-batch fermentations by controiied
intermitient feeding (Impoolsup et al., 1989). Plasmid stabiiity of recombinant S.
cerevisiae YN124/pLG669-z in fed-batch culture was ewnined by Hardjito et al. (1993).
The fraction of plasmid-containing ceh could be maintained consaint over the length of
one fed-batch culture, suggesting that fed-batch processes were better suited to produchg
recombinant proteins. Although the effects of cycfic environmentai changes on plasmid
stabiüty are dear, the reasons for those effects are not fuIiy undemood.
Newer bioreaction strategies are king developed continually to improve
productivity. One such example is the airlift bioreactor (Chisti, 1989) that is particuiariy
capable of micf~envirOxuuentaî cycling with a rnechanidy robust, simple, and scalabIe