3.4 Cell Density, Viability, and Morphology in FBB
Cells in the fibrous matrices were collected from the FBB at the end of
the continuous fermentation study and analyzed for their density,
viability, and morphology. In general, more cells were observed at the
lower parts closer to the entrance of the reactor with higher glucose
and lower butanol concentrations since cell metabolism would be faster
with higher concentrations of nutrients and lower concentrations of
inhibiting end products. Biofilms with dense and thick layers of
elongated rod-shaped cells covering the fibers in the fibrous matrices
were observed throughout the fibrous bed, as can be seen in the scanning
electron micrographs (Figure 7 ). The total cell density in the
FBB was approx. 100 ± 15 g cell dry weight/L reactor with
>70% cell viability. No obvious sporulation nor autolysis
was observed even for cells from the top part of the bioreactor,
suggesting that cells in the FBB remained healthy and were highly active
throughout the entire process over a period of more than
400~600 h. It should be noted that there were continuous
cell bleedings (~1.0 OD at 0.06 h-1and 0.12 h-1 and ~3.0 OD at 1.88
h-1) from the FBB throughout the continuous
fermentation, which allowed for continuous removal of old or stressed
cells to be replaced by new and more productive cells (Lewis and Yang,
1992). Furthermore, cells with the elongated morphology in the FBB were
adapted to better tolerate toxic metabolites such as butyric acid and
butanol. The elongated rod morphology with a higher specific surface
area could increase mass transfer and efflux for exchanges of nutrients
and metabolites between cells and the environment as previously reported
(Suwannakham and Yang, 2005; Zhu and Yang, 2003). The beneficial effects
of cell immobilization in the FBB including enhanced tolerance to toxic
metabolic end products and increased product titer, productivity and
yield have also been reported for other clostridia such as C.
tyrobutyricum (Jiang et al., 2011) and C. formicoaceticum (Huang
et al., 1998) and other bacteria such as propionibacteria (Zhang and
Yang, 2009).