Question

A patient goes to the doctor's office with symptoms of a urinary tract infection and provides 50 mL of urine. The urine sample is transferred to the lab where a technician makes a 10^-5 serial dilution (DF 100,000) in a total volume of 1 mL. Sketch/draw how you would prepare these dilutions from the starting culture including:

A) the volume of solute in each tube

B) the volume of solvent in each tube

Answer 1

Serial dilution refers to the method of diluting a stock solution where concentration decreases by the same quantity in each successive step. Dilutions are expressed as the ratio of the quantity of a desired solute (in this case, urine) contained in a solvent (diluent).

In order to prepare 10^-5 dilutions of the stock solution, the following procedure is used:

• Take 5 test tubes and label them A, B, C, D, and E.
• Add 9 ml of distilled water (solvent) to all the test tubes.
• Transfer 1 ml of stock solution to test tube A. Thus, here 1 ml of solute is diluted with 9 ml of solvent, making 1/10 dilution or 10^-1 dilution. Mix this solution well.
• From test tube A transfer 1 ml to test tube B. This dilution makes the concentration in test tube B 1/100^th dilution that is 10^-2 dilution. Mix the solution well.
• Similarly, 1 ml is transferred from test tube B to test tube C which makes the concentration in test tube C to be 1/1000^th dilution or 10^-3 dilution. Mix the solution well.
• Transfer 1 ml from test tube C to test tube D which makes the concentration in test tube D to be 1/10,000^th dilution or 10^-4 dilution. Mix the solution well.
• Transferring 1 ml from test tube D to test tube E makes the concentration in test tube E to be 1/10,0000^th dilution or 10^-5 dilution.
• Thus, in each step the stock solution has been diluted 10 times or each tube will be a 10-fold dilution starting from undiluted tube.

Serial dilution is a series of sequential dilutions used to reduce a dense culture of cells to a more usable concentration.