Bacterial Growth and Division
549
If one wished to state the growth law,
the growth law is simply the sum of the
individual synthetic patterns of the three
categories of the cell material.
12
Experimental Analysis of the Bacterial
Division Cycle
How can one study the pattern of synthesis
of cell components during the division
cycle? At Frst thought, it appears that one
could study the division cycle by preparing
a
synchronized
bacterial
culture – that
is, a culture composed of cells all of
the same age, for example, newborn
cells – and taking measurements of the
cell components at different times during
the division cycle. It turns out that this
method has rarely, if ever, led to any
concrete results. This is presumably due
to the introduction of perturbations and
artifacts when cells are synchronized, as
no naturally synchronized populations are
available.
The methods that have given results are
nonperturbing, nonsynchrony methods.
One is the membrane-elution method,
or ‘‘baby machine,’’ which analyzes the
division cycle of unperturbed cells. In this
method, a label is added to a growing
culture, the labeled cells are Fltered onto a
membrane, the cells bind as though bound
at one end or pole, and when medium is
pumped through the inverted membrane,
newborn cells are eluted from the bound
cells. Since the newborn cells arise from
the bound cells as a function of the original
cell age, the Frst newborn cells arise from
th
eo
ld
e
s
tc
e
l
l
sa
tth
et
im
eo
fl
ab
e
l
in
g
.
With time, the newborn cells arise from
cells labeled at younger and younger
cell ages. By measuring the amount of
label per cell during elution, one can
determine the pattern of synthesis during
the division cycle. This has been done
successfully for DNA, plasmids, protein,
RNA, peptidoglycan, and cell membrane.
Another method for cell-cycle analysis
is flow cytometry, where the amount of
material per cell in a cell population is
determined for individual cells by a com-
plex system of liquid handling and laser-
excitation engineering. The quantitative
distribution of cell material can give the rel-
ative rate of synthesis during the division
cycle. This method has been successful
for DNA replication during the division
cycle and has conFrmed the initial re-
sults obtained with the membrane-elution
method. It is not clear whether the method
is sensitive enough for bacteria to enable
the discovery of a particular pattern of DNA
replication.
13
Genetic Analysis of the Bacterial Cell Cycle
Complementing the physiological and bio-
chemical analyses of the bacterial division
cycle has been a large amount of work on
isolating and analysing mutants that affect
the division cycle. Because a successful di-
vision requires that the cell make all of
its required material, a mutation affect-
ing the synthesis of any required material
is, in some trivial sense, a mutation af-
fecting the division cycle. Thus, a mutant
unable to make a particular amino acid,
such as leucine, that is required for growth
would stop progressing through the divi-
sion cycle. However, this would be a trivial
relationship to the division cycle because
the leucine is required throughout the di-
vision cycle and is not related to a speciFc
control mechanism.
More interesting are those mutants that
affect the major events of the division cycle
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