Microbial Growth Website

IBERS: Institute of Biological, Environmental and Rural Sciences

Test Your Understanding: Answer

The correct answer is 1,900,000 (i.e. 1.9x10⁶) cells/ml.

The table below summarises the possible reasons for incorrect answers.  How to workout the answer is given below the table.

 

 

 

 

 

 

 

 

 

 

 

The correct answer is calculated as follows.

The dilution which gave between 30 and 300 colonies on the plate was the 10^-3 (1 in a 1000) dilution. This gave 190 colonies which means that the 0.1ml sample spread on the plate contained 190 single bacterial cells (which on incubation each gave a visible colony).

We need the number of cells/ml not per 0.1ml so we multiply by 10 to allow for this. Thus the sample spread on the plate contained 190 cells/0.1ml, i.e. 1,900 cells/ml.

The 1,900 cells/ml is for the 10^-3 (1:1000) dilution of the original sample. This means that the original sample was 1000 times more concentrated and so contained 1,900 x 1000 cells/ml. Thus, the original sample contained 1,900,000 (i.e. 1.9x10⁶) cells/ml.

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