You are given a flask of bacterial cell suspension with a concentration of 1.6 x

Question

You are given a flask of bacterial cell suspension with a concentration of 1.6 x 1012 cfu/ml. You have been asked to make a single dilution SERIES from which you select the appropriate dilutions and plate the appropriate volumes to produce plates with the following colony numbers: 320, 240, 180, 80, 40. You have been given 6 empty sterile test tubes and 50 ml of diluent. You may not exceed the number of tubes or the diluent volume. Draw out the dilution (as a figure or a grid) and provide information on which dilutions you will use and how much you will plate from the dilutions to obtain the required colony numbers. Use plating volumes that you can actually plate.

Explanation / Answer

Make 1 : 100 dilution

Put arounf 9.9 ml of diluent in tubes 1 -5. From master tube take out 100 ul of concentrate and pour into tube one. Likewise take 100 ul from next tube and pour in tube 2 and continue so on.

CFUs in tube 1 = 1.6 x 1010

CFUs in tube 2 = 1.6 x 108

CFUs in tube 3 = 1.6 x 106

CFUs in tube 4 = 1.6 x 104

CFUs in tube 5 = 1.6 x 102

Use tube 4 for getting required colonies by taking following amount

To get 320 colonies take about 200 ul (plating volume)

To get 240 colonies take about 150 ul (plating volume)

To get 180 colonies take about 112.5 ul (plating volume)

To get 80 colonies take about 50 ul (plating volume)

To get 40 colonies take about 25 ul (plating volume)

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