Online Textbook Bacteriology is continuously updated and includes information on Staphylococcus, MRSA, Streptococcus, E. coli, anthrax, cholera, tuberculosis, Lyme disease and other bacterial diseases of humans.
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The Online Textbook of Bacteriology is a general and medical microbiology text and includes discussion of staph, MRSA, strep, Anthrax, E. coli, cholera, tuberculosis, Lyme Disease and other bacterial pathogens.
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Tag words: bacterial growth, growth curve, lag phase, exponential growth, generation time, viable cell count, continuous culture.

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interests include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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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 constituents. It depends upon the ability of the cell to form new protoplasm from nutrients available in the environment. In most bacteria, growth involves increase in cell mass and number of ribosomes, duplication of the bacterial chromosome, 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 growth by binary fission.  Most bacteria reproduce by a relatively simple asexual process called binary fission: each cell increases in size and divides into two cells. During this process there is an orderly increase in cellular structures and components, replication and segregation of the bacterial DNA, and formation of a septum or cross wall which divides the cell into two progeny cells The process is coordinated by the bacterial membrane perhaps by means of mesosomes. The DNA molecule is believed to be attached to a point on the membrane where it is replicated. The two DNA molecules remain attached at points side-by-side on the membrane while new membrane material is synthesized between the two points. This draws the DNA molecules in opposite directions while new cell wall and membrane are laid down as a septum between the two chromosomal compartments. When septum formation is complete the cell splits into two progeny cells. The time interval required for a bacterial cell to divide or for a population of bacterial cells to double is called the generation time. Generation times for bacterial species growing in nature may be as short as 15 minutes or as long as several days. Electron micrograph of Streptococcus pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller University.

For unicellular organisms such as the bacteria, growth can be measured in terms of two different parameters: changes in cell mass and changes in cell numbers.

Methods for Measurement of Cell Mass

Methods for measurement of the cell mass involve both direct and indirect techniques.

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 consumption, etc.

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 their numbers. The turbidity or optical density of a suspension of cells is directly related to cell mass or cell number, after construction and calibration of a standard curve. The method is simple and nondestructive, but the sensitivity is limited to about 107 cells per ml for most bacteria.

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Kenneth Todar is an emeritus lecturer at University of Wisconsin-Madison. He has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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