548
Bacterial Growth and Division
surface cannot outpace or lag behind cy-
toplasm synthesis. DNA synthesis cannot
go faster because it is waiting to initi-
ate new rounds of DNA replication in
response to cytoplasm synthesis nor can
it go slower because the cytoplasm would
increase without initiating new rounds of
DNA replication. Cell surface is made to
just enclose the newly synthesized cyto-
plasm. The regulation of surface and DNA
synthesis by cytoplasm thus explains why
there is no dissociation of these syntheses
during the division cycle and during the
growth of a culture. Furthermore, the fail-
uretocompleteDNAreplication(sothata
cell has only one genome) prevents cell di-
vision; the cell thus ensures that there will
be enough material (genome, cytoplasm,
and cell surface) to allow the production
and survival of two new daughter cells.
11
The Bacterial Growth Law during the
Division Cycle
It has been speculated that there is
a ‘‘bacterial growth law’’ that can be
discovered
by
sensitive
methods
of
analysis. Does the cell grow linearly,
bilinearly, exponentially, or perhaps follow
some other, yet undiscovered, growth law?
If there were a general law of cell growth,
independent of the biosynthetic patterns
of the three categories that comprise the
cell, then the individual categories of
biosynthesis would have to accommodate
themselves to this overall growth law.
There is no simple mathematical growth
law regulating or describing bacterial
growth during the division cycle. Bacterial
growth during the division cycle is the
simple weighted sum of the biosynthetic
processes that are described by the three
categories proposed here. As presented
in
Table 1,
it
can
be
seen
that
the
components of
the
cytoplasm
of
the
cell are 80% of the total cell weight.
This means that the growth of the cell
is approximately exponential. A slight
deviation from exponential growth is due
to the contribution of cell surface and DNA
synthesis. Cell growth, then, is the result
of a large number of individual reactions,
regulated by local conditions, and not
conforming to any overriding growth law.
Tab. 1
Composition of
Escherichia coli
, a typical gram-negative,rod-shapedbacterium.
Cell component
Molecules per cell
Percent dry weight
Cytoplasm
Protein
2,350,000
55.0
RNA
255,480
20.5
Polyamines
6,700,000
1.1
Glycogen
4,300
3.5
Metabolites, ions, cofactors
2.5
Surface
Peptidoglycan
1
2.5
Lipid
22,000,000
9.1
Lipopolysaccharide
1,430,000
2.4
Genome
DNA
2.1
3.1
previous page 548 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 550 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off