Serial Dilution Method

A serial dilution is the stepwise dilution of a substance in solution. Usually the dilution factor at each step is constant, resulting in a geometric progression of the concentration in a logarithmic fashion. A ten-fold serial dilution could be 1 M, 0.1 M, 0.01 M, 0.001 M... Serial dilutions are used to accurately create highly diluted solutions as well as solutions for experiments resulting in concentration curves with a logarithmic scale. A tenfold dilution for each step is called a logarithmic dilution or log-dilution, a 3.16-fold (100.5-fold) dilution is called a half-logarithmic dilution or half-log dilution, and a 1.78-fold (100.25-fold) dilution is called a quarter-logarithmic dilution or quarter-log dilution. Serial dilutions are widely used in experimental sciences, including biochemistry, pharmacology, microbiology, and physics.
source:http://en.wikipedia.org/wiki/Serial_dilution

source; http://sciencefair.math.iit.edu/techniques/SerialDilution/
source; http://www.sigmaaldrich.com/analytical-chromatography/microbiology/learning-center/theory/introduction.html
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In microbiology, colony-forming unit (CFU or cfu) is a measure of viable bacterial or fungal numbers. Unlike direct microscopic counts where all cells, dead and living, are counted, CFU measures viable cells. For convenience the results are given as CFU/mL (colony-forming units per milliliter) for liquids, and CFU/g (colony-forming units per gram) for solids.
The theory behind the technique of CFU is to establish that a single bacterium can grow and become a colony, via binary fission. These colonies are clearly different from each other, both microscopically and macroscopically. However, some bacteria do not separate completely during the sample preparation process (Staphylococcus, Streptococcus) and the results of the count will be below the number of individual cells using direct methods.
source; http://en.wikipedia.org/wiki/Colony-forming_unit

CLICK ON IMAGE TO ENLARGE VIEW.  source; http://www.mansfield.ohio-state.edu/~sabedon/biol4035.htm
source; http://people.rit.edu/~gtfsbi/IntroMicro/20101IntroMicroLab2.htm

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