A culture of E. coli is diluted 1:1000. 1 ml of the 1:1000 dilution is spread onto an agar plate. After incubation, 50 colonies are observed. What is the concentration of the original culture? Show your work and include proper units for full credit.

5.0 x CFU/mL

Explanation:

The dilution factor for a 1:1,000 dilution is .

Since 1 mL of the dilution was taken, the dilution factor that yielded the colony is /1 =

In order to find the colony forming unit (CFU) per volume (1 mL) of the original culture, divide the number of colony resulting from the plating of 1 mL of the dilution by the dilution factor. Hence;

CFU/mL of the original culture = 50/

          = 5.0 x

Therefore, the concentration of the original culture is 5.0 x CFU/mL.

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