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You count 45 colonies of bacteria on a media plate. The culture was made by plating...

You count 45 colonies of bacteria on a media plate. The culture was made by plating 1/10 of a mL of culture media that has already been diluted by a factor of 10,000. What is the best estimate for the density of bacterial cells in the starting (undiluted) culture?

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Answer #1

The plate has 45 colonies.

The plate was made by using 0.1 mL of diluted culture and therefore 0.1 mL of diluted culture has 45 CFU. In other words the diluted culture has 450 CFU/mL.

Since this media is already diluted 10,000 times the concentration of the undiluted culture is 10,000 times the concentration of the diluted culture.

Therefore, Concentration (Undiluted culture) = Concentration (Diluted Culture) * Dilution factor = 450 CFU/mL * 10,000 = 4,500,000 CFU/mL

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