THE VIRTUAL EDGE: Lab 5 Cultivation of Bacteria I

In step 3, the product of the individual dilution factors is calculated to give the final dilution factor:

Step 3: 1 X 10^-2 * 1 X 10^-2 * 1 X 10^-1 * 1 X 10^-1 = 1 X 10^-6 (final dilution factor)

In microbiology, the reciprocal of the final dilution factor is called the plating factor.  In step 4, the plating factor is calculated.

Step 4: plating factor (p.f.) = 1/(1 X 10^-6)

Plating factor (p. f.) = 1 X 10⁶

Finally, to determine the concentration of culture A, the average number of colonies (as calculated in method 1) is multiplied by the plating factor:

Step 5: p. f. * average # of colonies = titer of original culture

(1 X 10⁶) * 98 = 9.8 X 10⁷ bacteria/mL

**As can be seen, this is the same number that was determined using method 1!**

Please feel free to use either method 1 or 2.  Method 1 is more concise and more clearly tracks units.  Unfortunately, in microbiology method 2 is commonly used and even if you select method 1 it will be necessary to be familiar with the plating factor term. 

* Significant figures: The greatest number of significant figures that can be known is three, as it is impossible to count halves of colonies.  However, less than three significant figures may be retained depending on the number of colonies and the accuracy of the measuring devices selected.

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Lab 5 / Aseptic Technique / Nutritional Requirements for the Cultivation of Bacteria / Enumeration of Bacteria / Dilutions / Lab 5 Organisms

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