Bacterial Growth and Division
539
Fig. 4
Cell-surface synthesis during the
division cycle. Before invagination, the
cell grows only by cylinder extension.
Each cell is drawn to scale, with the
volume of the cells increasing
exponentially during the division cycle.
The shaded regions of the cell indicate
the amount and location of wall growth
(whether in pole or sidewall) each tenth
of a division cycle. The width of the
shaded area is drawn to scale.
Cell-surface growth actually occurs
throughout the sidewall and not in a
narrow contiguous zone. The variable
locations of wall growth in this Fgure
have no speciFc meaning, but are meant
to show synthesis occurring all over the
sidewall during the division cycle. Before
invagination, the ratio of the surface
increase to volume increase is constant.
When pole synthesis starts, at age 0.5 in
this example, there is an increase in the
ratio of the surface increase to the
volume increase. Any volume not
accommodated by pole growth is
accommodated by cylinder growth. At
the start of pole growth, there is a
reduction in the rate of surface growth
in the cylinder. As the pole continues to
grow, there is a decrease in the volume
accommodated by the pole and an
increase in the rate of growth in the
sidewall.
0.0
0.1
0.2
0.3
0.4
0.7
0.8
0.9
1.0
0.5
0.6
Cell age
As with cytoplasm and DNA synthesis,
we can derive the cell-cycle pattern of
surface synthesis from our understanding
of the molecular aspects of peptidoglycan
synthesis. The peptidoglycan sacculus of
the cell is composed of glycan strands
encompassing the cell, perpendicular to
the long axis, as shown in Fig. 5. These
strands are cross-linked by peptide chains.
Because the cross-linking in the load-
bearing layer is not complete, one can
have new strands in place, below the taut
load-bearing layer, prior to the cutting of
the bonds linking old strands. As shown in
Fig. 5, the load-bearing layer is stretched
by the turgor pressure in the cell. An
in±nitesimal increase in cytoplasm leads
to an in±nitesimal increase in the turgor
pressure of the cell. This increase in turgor
pressure places a stress on all of the
peptidoglycan bonds, thus reducing the
energy of activation for bond hydrolysis.
The result is that there is an increase in
the cutting of stressed bonds between the
glycan chains. When a series of cuts is
made, allowing the insertion of a single
new strand into the load-bearing layer,
there is an in±nitesimal increase in cell
volume. This increase in volume leads to
a reduction of the stress on the remaining
bonds. By a continuous repetitive series
of cytoplasm increases, surface stresses,
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