You count 47 cfu on a spread plate. The plate was prepared by spreading 0.2 ml of a 1:10,000 dilution of the original sample. What is was the concentration of the original culture (in cells/ml)?

2.35 x 10^6 cfu/ml

Explanation:

Concentration of the original culture: ?

Dilution factor: 10^4

Colony count per plate: 47cfu

Volume of cultured plate: 0.2ml

Concentration of the original culture= Colony count per plate x Dilution factor

                                                                            Volume of cultured plate

Concentration of the original culture = 47 x 10^4  

                                                                    0.2

Concentration of the original culture = 2.35 x 10^6 cfu/ml

You can further take the log of your answer which would be:

Log(2.35 x 10^6) = 6.371

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