bioreactor studies of heterologous protein production by ...

5) Slower growth rate of cells may M t opportnbities for plasmid loss (nickinger and
Rouse, 1993).
These suggestions regardjng possible mechanisms are largely speculative. A sound
demonstration of the mechanisrn of stabilïzation at the molecular or ceiiuiar bels remains
to be accomplished. In fact, enhanced plasmid retention upon irnrnobilization may not
have a single explanation; multiple interactions are more likely to be responsible.
2.4. Immobilization Methods and Inmiobilized Cd Bioreactors
Choice and design of immobilued cell bioreactors depends, to large extent, on cell
immobilization materials and methods. Before discussing immobilized ceii bioreactoa, it
is necessary to give a brief review of yeast ceii immobiiization.
Research in alcohol fermentation using immobkd yeast ceUs has ben carried out
for many years. Many materials and methods have been developed to immobiüze yeast
ceiis. One commoniy used method is gel enmpment DHerent types of gel matrices were
used to entrap yeast ceiis, including calcium alginate (Kierstan and Bucke, 1977), agar
(Kuu and Polack, 1983), carrageenan (Wada et al., 1980) and polyacryiamide (Couderc
and Baratti, 1980). There an two major drawbacks reïating to gel entrapment methods.
One is the instability problem of gel particles. The growth of œUs and the evolution of
carbon dioxide wiil weaken and disupt the gel matrices. The other is the severe
intraparticle mas transfer limitations, which causes celis hide the gel to be practicaiiy