bioreactor studies of heterologous protein production by ...

6.3. Repeated Batch Fermentation in lnunobih'zed-CeIl BioreactOr
By using yeast film as resident inocuiurn, repeated batch fermentations were perfonned in
the immobilizedceil-frlm airiift bioreactor to further investigate the effect of
immobilization. nie detailed proceduns for the repeated batch fementation was
described in Section 3.5.4. Resuits for two repeated batch fermentation experiments are
shown in Figure 6.12. Good reproducibiiity was obtained between the two experiment
mns. Each batch Iasted 24 hours and the immobiiized-ceU bioreactor was run for two
weeks (14 batches). Over the two week period of fermentation, both the fraction of
piasmid-beuing celk and enzyme concentrations were nearly constant Stable enzyme
production performance was achieved. h contrast, in free suspension repeated batch
fermentations significant plasmid loss and enzyme production decay were observed. As
show in figure 6.13. both the fraction of plasmid-bearing cells and enzyme concentration
decreased over the culture penod.
Although the continuous studies showed thot the plasmid los and the decg of the
enzyme production were reduced in the imrnobilïzed ceii system. the instability stül
occurred (see Figures 6.7 and 6.8). By openring the immobüised-ceil-fi bioreactor in
repeated batch mode, the stabiiity of the recombinant yeast w u further irnproved. This is
the result of maintahhg a higher proportion of plasmid-beaîng celis Vi the immobiüzation
matrix. As discussed in the Section 6.2.3, the yeast 61m could be a dynamic reserve of
highly concentrated plamid-bearîng ceUs having a high plaunid copy nurnber and Iess
sebmgarionai instability, therefore providing for an enhanced gertetic stability. In the