3. A researcher sets up a serial dilution as shown below. Starting with a satura

Question

3. A researcher sets up a serial dilution as shown below. Starting with a saturated culture, she performs 3 dilutions (1D, 2D, and 3D) and plates 0.1 mL from both 2D and 3D dilutions onto LB plates prior to overnight incubation at 37 degree C. 3a. The following day, the researcher counts 120 colonies on the plate from the 3D dilution. Based on this information, what is the concentration of the original bacteria culture in cells/mL? Write your final answer in scientific notation, and show your work. (15pts) 3b. Based on the colony number for plate 3D, how many colonies are expected on plate 2D? Show your work. (3pt)

Explanation / Answer

3a. CFU/ml = (No of colonies x dilution factor)/ volume of culture plate

Here plate 103 has yielded 120 colonies in 0.1 ml of sample

Hence, Bacteria/0.1 ml of sample = 120 x 103 = 1.2 x 105 Cells/100 micro liter (or) 1.2 x 106 cells/ml.

Here dilution factor = 0.1 x 0.01 x 0.01 = 10-4

Hence 1.2 x 103 cells/ ml is present in the sample.

3b. By Substituting the Eqn we know that the CFU is 1.2 x 103, The dilution factor for 102 plate is = 0.1x0.01 = 0.001; the culture volume = 0.1

1.2 x 103 = (No of colonies x 0.001) / 0.1

No of colonies x 0.001 = 120

No of colonies = 1.2 x 105

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