How Is Bacteria Measured?


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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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The Growth of Bacterial Populations (page 2)

(This chapter has 4 pages)

© 2008 Kenneth Todar, PhD

Methods for Measurement of Cell Numbers
Measuring techniques involve direct counts, visually or instrumentally, and indirect viable cell counts.

1. Direct microscopic counts are possible using special slides known as counting chambers. Dead cells cannot be distinguished from living ones. Only dense suspensions can be counted (>107 cells per ml), but samples can be concentrated by c

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