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The Growth of Bacterial Populations (page 1)
(This chapter has 4 pages)
© Kenneth Todar, PhD
Measurement of Bacterial Growth
Growth is an orderly increase in the quantity of cellular
It depends upon the ability of the cell to form new protoplasm from
available in the environment. In most bacteria, growth involves
in cell mass and number of ribosomes, duplication of the bacterial
synthesis of new cell wall and plasma membrane, partitioning of the two
chromosomes, septum formation, and cell division. This asexual process
of reproduction is called binary fission.
Figure 1. Bacterial
by binary fission. Most bacteria reproduce by a relatively simple
asexual process called binary fission: each cell increases in
and divides into two cells. During this process there is an
increase in cellular structures and components, replication and
of the bacterial DNA, and formation of a septum or cross wall which
the cell into two progeny cells The process is coordinated by
bacterial membrane perhaps by means of mesosomes. The DNA
is believed to be attached to a point on the membrane where it is
The two DNA molecules remain attached at points side-by-side on the
while new membrane material is synthesized between the two
This draws the DNA molecules in opposite directions while new cell wall
and membrane are laid down as a septum between the two chromosomal
When septum formation is complete the cell splits into two progeny
The time interval required for a bacterial cell to divide or for a
of bacterial cells to double is called the generation time.
times for bacterial species growing in nature may be as short as 15
or as long as several days. Electron micrograph of Streptococcus
pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with
The Laboratory of
Pathogenesis and Immunology, Rockefeller University.
For unicellular organisms such as the bacteria, growth can be
in terms of two different parameters: changes in cell mass and
in cell numbers.
Methods for Measurement of Cell Mass
Methods for measurement of the cell mass involve both direct and
1. Direct physical measurement of dry weight, wet weight, or
volume of cells after centrifugation.
2. Direct chemical measurement of some chemical component of
the cells such as total N, total protein, or total DNA content.
3. Indirect measurement of chemical activity such as rate of
O2 production or consumption, CO2 production or
4. Turbidity measurements employ a variety of instruments to
determine the amount of light scattered by a suspension of cells.
Particulate objects such as bacteria scatter light in proportion to
numbers. The turbidity or optical density of a suspension of
is directly related to cell mass or cell number, after construction and
calibration of a standard curve. The method is simple and
but the sensitivity is limited to about 107 cells per ml for