Calculation of Bacterial Cells in a solution

Question

I have completed an experiment where i have 32 colonies in an agar plate (volume of 100ul) , with a dilution of 10^-4

How would i work out the cell concentration of the original suspension?

Answer 1

To get the original concentration of bacteria, first divide the number of colonies by the volume plated, then multiply by the dilution factor. That assumes that each colony is derived from a single cell. This is statistically valid for plates with between 30 and 300 colonies, so no problem with the data.

The plating volume was 1000 microliters, or 1 milliliter, so there were 32 cells per milliliter in the plating suspension. That in turn was diluted by 10^-4 prior to plating. The dilution factor is the inverse of the dilution, so in this case it would be 10⁴. Multiply that by the concentration that was plated, and you have 32 x 10⁴, = 3,2 x 10⁵ cells per milliliter in the original suspension.

Hope this helps.