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Skip to 0 minutes and 10 seconds Serial dilution is a technique which students find quite hard to complete and to understand the maths which is linked to it. It is a technique often used in microbiology to estimate the population size in a sample. We are going to look at 10 fold dilution.

Skip to 0 minutes and 29 seconds Take 5 sterile test tubes and label with fractions: 1/10, 1/100, 1/1,000, 1/10,000, 1/100,000. Into each of your labelled test tubes put 9cm3 of distilled water. Then take the sample you wish to dilute and carefully measure out 1cm3. We’ve added food colouring to the sample to make the dilution easier to see. Transfer it to the first test tube labelled 1/10.

Skip to 1 minute and 6 seconds Mix the sample thoroughly by filling and emptying the syringe or pipette several times. Your first dilution is mixed. You will now use this to create the next dilution. With a clean pipette take 1cm3 of the mixed contents of the first tube and put it into the second tube labelled 1/100 and mix this thoroughly.

Skip to 1 minute and 34 seconds Repeat the process of removing 1cm3 of sample and adding it to the next tube until you have made your fifth dilution. Use a clean pipette each time. Getting your students to develop this technique will enable them to feel confident when carrying out microbiology practicals.

Teaching practical skills

In this video we are looking at the technique of serial dilution. This is a skill which is used in microbiology to dilute samples down to allow us to estimate the number of bacteria in a sample or can be used to produce a range of dilutions to compare a test sample against for semi quantitative analysis.

Although in this video we have used sterile test tubes, if this techniques is not being used for a microbiology practical the test tubes do not need to be sterilised.

As you watch the video write a list of the skills the students need to be able to complete this practical activity

What are the stages involved in developing these skills e.g. reading a scale on a pipette, measuring cylinder. Consider the following questions:

  1. When would you teach this to students?
  2. How would you teach it?
  3. How would you develop the skill?


Select one of the skills you have listed from watching the video and answer the three questions above.

Share your ideas by posting them in the comments below.

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This video is from the free online course:

Teaching Practical Science: Biology

National STEM Learning Centre