Civil Engineering Reference
In-Depth Information
their survival and binary i ssion. h is also helps the bacteria to protect itself from other
organisms, heavy-metal ions, and UV radiation [10]. h e dif erent steps involved in
cellulose synthesis are polymerization of the glucose molecules into linear β-1-4 glucan
chain polymer within the cell, extrusion of the polymer chains through pores in the
bacterial envelope, formation of l at ribbon-shaped microi brils by lateral aggregation
of glucan chains outside the cell (Figure 17.2a). h e cellulose producing bacteria con-
sist of ~ 50 to 80 linearly arranged single row of pore-like structures on the exterior
surface of lipolpolysacharide (LPS) envelope (Figure 17.2b). h ese pores, which are
believed to be comprised of LPS-protein complexes, have crater-like ring wall struc-
ture of diameter ~ 12 to 15 nm with central hole of ~ 3.5 nm diameter. h is row of
pores on the LPS envelop have corresponding axially aligned row of particles of size
~ 10 nm associated with periplasmic space and plasma membrane. h ese particles are
assumed to be the glucan chain synthesizing sites made up of multi-enzyme complexes
called cellulose synthase. During the synthesis process each of these particles produces
9 to 15 glucan chains simultaneously. h e pore corresponding to each particle crystal-
lizes these glucan chains into tactoidal aggregates of thickness ~ 1.5 to 3.5 nm having
let -handed triple helical structure and extrudes them into the space exterior to the
cell. h ese tactoidal aggregates have been found to be the nondisociable smallest units
of cellulose microi brils. h e microi brils or aggregates from dif erent pores coalesce
laterally in hierarchical manner to form l at cellulosic ribbon in the space exterior to
the cell envelope. It has been experimentally found that the rate of polymerization of
glucan chain within the cell and the rate of formation of l at ribbons outside the cell are
perfectly synchronized processes. Even though most of the steps involved in cellulose
microi bril formation except polymerization happen in the space immediately exterior
to the cell surface, the i nal orientation of the cellulose ribbons formed by aggrega-
tion of microi brils depends on the spatial arrangement of the synthesizing sites on the
(
a
)
Cellulose
synthesizing sites
External surface
of Bacteria
Tactoidal aggregates
(
b
)
LPS protein
Lipopolysacharide
Peptidoglycan
Periplasmic space
Plasma membrane
LPS
LPS
PPG
PPS
PM
PPG
PPS
PM
Particles made of
cellulose synthase
Cytoplasam
Figure 17.2
(a) Schematic showing the synthesis mechanism of cellulose by acetobacter xylinum at
the cell surface showing the dif erent hierarchical stages involved; (b) enlarged view of the cellulose
synthesizing sites that are present on the bacterial surface.
