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calculating_cfu - MIC102L Spring 2009 CALCULATING CFU MIC...

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MIC102L Spring 2009 CALCULATING CFU MIC 102L Fall 2009 There are a number of different strategies for determining the CFU (colony forming units) in a liquid or solid sample. Some students might find the following explanation and definitions helpful. Adapted from http://www.science-projects.com/serdil.htm Hypothetical Example: In the following, a bottle contains an unknown concentration of bacteria ("CFU/ml" means "colony forming units per milliliter", where each CFU equals one LIVE bacterium). One milliliter from the bottle is transferred into a tube containing 9 ml of water or saline (0.15% NaCl is called "normal saline" or "NS"). Thus the concentration of the bacteria in the tube is exactly 1/10 that in the bottle. And if 1 ml of that is transferred into a second tube of 9 ml NS, another 10-fold dilution is accomplished. It is now 1/100 or 10 -2 the concentration in the bottle. The next dilution is made by removing 1 ml from the second tube and transferring it to 9 ml NS in the third tube, etc.
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