Question

You are trying to determine the approximate bacterial count in a sample of soil. You weigh out 1 gram of soil and add it to 9ml water (tube 1). You then take 1 ml out of tube 1 and place it in 9ml of water in tube 2. Finally, you take 1 ml out of tube 2 and place it into 9 more ml of water in tube 3. You plate out 1.0 ml of the contents of tubes 2 and 3 onto PCA, incubate both for five days, and observe the results. Using the values given in the table below, calculate the CFUs (colony forming units) in the original soil sample. (You only need to use one tube for the calculation)

Plate contents	Number of colonies observed
Tube 2	431
Tube 3	52

Answer 1

The above experiment is serial dilution technique of pure culture isolation, from the tube 2 (1/100th dilution) and tube 3 (1/1000th dilution). By counting the CFU we can get the viable count of the bacteria after palting.

Colony forming units CFU is calculated by the formula :-

CFU = No of colonies in the Culture plate x Dilution factor / Volume of the culture plate

   = 52 x 10^ 3 / 25 [ generally the agar plate is 25ml]

= 2.08 x 10^ 3 CFU/ml