Marco is given an immortal cell line of epithelial cells to dilute. The initial concentration is 12.5 million cells. If he does four 1:10 serial dilutions, what will be the concentration of the last serial dilution

12.5 x 10^6/10000 = 1250 cells.

Explanation:

A serial dilution is the step-by-step reduction of the concentration of a solution. Usually, the dilution factor at each step is constant, resulting in a geometric progression of the concentration, which is logarithmic. Serial dilutions are used to create very dilute solutions by accurately reducing the concentration of a substance.

It is usually carried out in experiments where highly dilute solutions are needed, for example, those involving logarithmic scale concentration curves or those used to determine the density of bacteria. To do this, prepare several test tubes with, for example, 9 ml of dilution liquid and first add 1ml of the undiluted sample to the first tube and then make serial dilutions in the following tubes. The first tube will contain a 1:10 dilution, the second 1:100, the third 1:1000, the fourth 1:10000 and so on.

Use 1 ml of undiluted (stock) solution with a pipette into the first test tube, containing 9 ml of a liquid, which can be for example water, Phosphate-buffered saline (PBS) and mix. This is the 1:10 solution. For the second dilution, take 1 ml of the dilution from the 1:10 tube and add it to the 9 ml of liquid to make the  1:100 tube and mix. The third tube is prepared with 1 ml of the 1:100 solution and 9 ml of liquid and this will be the 1:1000 dilution. The last one is prepared with 1 ml of 1:1000 and 9 ml of liquid and it will be 1:10000 (the fourth dilution).

So, if you start with 12.5 million cells and dilute four times with serial dilutions of 1:10 each, the final concentration will be four times lower:

12.5 x 10^6/10000 = 1250 cells.