528
Bacterial Growth and Division
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A dividing bacterial cell must, on average, have precisely twice as much of everything
found in a newborn cell. How does a bacterial cell ensure that all cell components
are duplicated between divisions, and how does a cell ensure that it does not divide
prior to duplicating all of its components? These questions have been investigated by
studying biosynthesis of cell components during the division cycle. Cell cytoplasm
increases uniformly and exponentially during the division cycle. There does not
appear to be any cell-cycle-speciFc syntheses of cytoplasmic components during the
division cycle of a well-studied bacterium such as
E. coli
. DNA replication is initiated
at a speciFc time in the division cycle when the cell mass per chromosome origin
reaches a particular value. Once initiated, DNA replicates at a constant rate such that
the time between initiation and termination of replication is relatively independent
of the growth rate. The time between termination of replication and cell division
is also relatively constant. High-copy plasmids replicate throughout the division
cycle and low-copy plasmids replicate at a particular time during the division cycle
according to rules similar to that for initiation of chromosome replication. The cell
surface grows in response to the increase in cell mass so that the turgor pressure
inside the cell, and the cell density, are constant during the division cycle. At
division, the components of the cell are segregated to the two daughter cells. The
cytoplasm segregates randomly, the DNA segregates stochastically but nonrandomly,
and the peptidoglycan segregates in a manner consistent with the Fxed location of
the synthesized peptidoglycan. The key ‘‘events’’ during the division cycle are the
initiation and termination of DNA replication and the initiation and termination of
pole formation. There do not appear to be other cell-cycle-speciFc events or syntheses
during the division cycle that are related to the regulation of the division cycle.
1
Growth of Bacterial Cultures
1.1
The Classical Growth Curve
±or almost a century (since Buchanan,
1918), the classic growth curve has been
used as a description of the pattern of
bacterial growth. An overgrown culture is
diluted into fresh medium and at Frst there
is a ‘‘lag’’ phase, experimentally deFned
as a period before viable cell number
begins to increase. Then cell numbers
begin to increase and the culture enters
the ‘‘log’’ phase. The log phase is the
period of exponential increase in cell
number. Although the cells during this
period of steady state cell number increase
are growing exponentially, the cells are
generally referred to as being in ‘‘log’’
phase because of the usual plotting of cell
number on semilogarithmic coordinates.
During the log phase, plotting cell number
increase on semilogarithmic paper yields a
straight line that can be used to determine
the doubling time of a culture. After
the cells grow to higher concentrations,
cell growth slows as the cells enter the
stationary phase. Cell growth then ceases.
±inally, if a culture is studied long enough,
cells begin to lose viability or the ability to
form colonies, and the culture enters the
death phase.
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