Bacterial Growth and Division
529
During these phases, there are changes
in cell sizes. The initial cells in the
overgrown culture are small, cell size
increases during the lag phase, cells are
largest during the log phase, and then cell
size decreases as cells enter the stationary
phase. This classical view of the way in
which cells grow is really a special case of
the phenomenon of changes in cell size
with changes in growth rate.
1.2
Bacterial Growth at Different Growth Rates
It is possible to grow a given bacterium in
cultures at a wide range of growth rates.
For example, it is possible to grow
E. coli
with a 300-min interdivision time with
serine as the sole source of carbon, and a
little faster than 20-min interdivision times
in various complex broths available for
bacterial culture. By varying the medium
composition, it appears that any possible
growth rate is allowable, so that there is
a continuous range of allowable growth
rates between the extremely slow and the
extremely fast.
When bacteria are grown at a given
temperature
in
different
media
at
a
number of different growth rates, a regular
pattern of cell composition and cell size
is observed. The faster a cell grows
(richer medium) the larger the cell and
th
em
o
r
eth
eDNA
,RNA
,p
r
o
t
e
in
,an
d
other components per cell (Fig. 1). When
cell components are accurately measured,
it is found that the material per cell
increases exponentially as a function of
the reciprocal of the doubling time (i.e.
growth rate, as a shorter doubling time
indicates a faster growth rate).
1.3
The Shift-up
During a shift-up from a slow growth
medium (e.g. minimal medium, in which
cells are small) to rapid growth in a rich
medium (e.g. nutrient broth, in which
cells are larger), there is a regular pattern
of change in cell size and composition.
Immediately after the shift-up, there is
a rapid change in the rate of RNA and
protein synthesis to the new rate of
increase. There is a slower change in
the rate of DNA synthesis. There is a
delay before there is any change in the
rate of cell number increase. Because of
Fig. 1
(a) Composition of bacteria as a
function of growth rate. (b) Synthesis of
cell components during a shift-up from
minimal medium to faster growth in
richer medium.
Doublings per hour
0
1
2
RNA or
protein
DNA
Time [hours]
1
Cell number
Relative amount per mL
Shift-up
1
10
(b)
1
2
3
RNA
Relative amount per cell
1
10
Interdivision time [min]
60
30
20
Mass
DNA
Protein
(a)
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