Plate Count Method: Calculating Concentration of Viable Cells

How can we calculate the concentration of viable cells in a culture using the plate count method?

Given that 100 microlitres were plated of a 0.001 D culture, and 50 colonies were observed, what is the concentration of viable cells in the original culture?

Answer:

The concentration of viable cells in the original culture, after counting 50 colonies from a 0.001 dilution and plating 100 microliters, is 500,000 cells/mL.

To calculate the concentration of viable cells in the original cell culture from a plate count, you need to consider the dilution factor and the volume plated. In this scenario, 100 microliters (which is 0.1 mL) of a 0.001 or 1:1000 dilution culture was plated, and 50 colonies were observed. The dilution factor is 1/0.001 or 1000, and since we plated 0.1 mL, we need to multiply the number of colonies by 10 (to account for 1 mL) and then again by the dilution factor to find the original concentration. Therefore, the calculation would be 50 colonies × 10 (for 1 mL) × 1000 (dilution factor), which gives us 500,000 cells/mL.