530
Bacterial Growth and Division
the dissociation of mass increase and cell
number increase, the average cell size
increases during this Frst part of the shift-
up. After a period of time, there is a sharp
change in the rate of cell increase from the
old rate to the new rate. This occurs 60 min
after the shift-up (±ig. 1). Because there is a
lag in the change in the rate of cell division,
while cytoplasm synthesis increases to the
new rate almost immediately, during the
shift-up period there is an increase in cell
size producing the larger-sized cells in the
faster growth medium.
1.4
Classical Growth Pattern Is Merely a Series
of Shift-ups and Shift-downs
The classical growth pattern for a culture
can now be seen as a series of shift-ups and
shift-downs. When an overgrown culture
(nongrowing, and thus of essentially zero
growth rate) containing small cells is
diluted into fresh medium, there is a
‘‘shift-up’’
in
growth
rate.
The
mass
increases immediately with no cell division
occurring. The ‘‘lag’’ phase is thus one of
a lag in cell number increase, but there
is essentially no lag when considering
mass increase. After the normal cell size is
achieved for that medium, both the mass
and the cell number increase in parallel.
The rate of cell increase is determined by
the medium supporting the more rapid
growth. There is a constant larger-size cell
during exponential growth. The reverse
occurs as growth begins to slow down and
the cells approach the stationary phase.
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Frst, followed later by a cessation in the
increase in cell number. Because division
continues after mass increase ceases, the
average cell size decreases in an overgrown
culture.
2
The Bacterial Division Cycle
2.1
Cell Age and the Age Distribution during
Balanced Growth
By convention, a newborn cell has an age
of 0.0 and grows during the division cycle
to divide at age 1.0. The cell ages during the
division cycle are thus numbers between
0.0 and 1.0 indicating where, in the
division cycle, a cell is located at a particular
instant. A cell half-way between birth and
division is age 0.5. Cell age, by deFnition,
increases linearly during the division cycle.
However, cells at the same absolute time
after birth are only approximately the same
cell-cycle age because of the variability in
the absolute time required for the division
cycle of an individual cell.
In a growing culture, the distribution
of cell ages is not uniform. There are
twice as many newborn cells as dividing
cells. The age distribution is given by the
formula
N
=
2
1
x
,wh
e
r
e
x
is the cell
age between 0.0 and 1.0. This formula
indicates that there are twice as many
newborn cells (
x
=
0
.
0) as dividing cells
(
x
=
1
.
0). If all cells grew with exactly
the same interdivision time, this formula
would give the age distribution exactly.
However, because of cycle variability there
is a smoothing of the function and the
actual distribution is an approximation
of the theoretical distribution. Because
of the age distribution, the properties
of an exponentially growing culture are
independent of time. This means that all
patterns of synthesis within the division
cycle give an exponentially increasing
amount of material in a growing culture.
This is because the age distribution is
constant during steady state, exponential
growth.
Thus,
whether
something
is
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