Serial dilutions
Using a serial dilution in microbiology is a good way to get easy numbers of bacterial colonies
to count on a plate. It is the stepwise dilution of a substance of your bacteria. With each
dilution the concentration of bacteria decreases by a factor of 10.
Equipment needed:
Sterile bijoux containing 9ml of sterile water
1ml pipette
Bacterial culture.
Making Serial dilutions:
1. Inoculate your first bijoux with your environmental swab. Snap the swab into the
water, replace the lid and mix the sample well. This is you NEAT dilution.
2. Remove 1ml of NEAT from the bijoux and transfer it to a new bijoux of 9ml sterile
water. Replace the lid and mix well. Now your have produced your 1 in 10 (1:10)
dilution.
3. Repeat step 2 for as many dilutions as you require (4 dilutions should be enough).
Remember, with each dilution you are decreasing your concentration of bacteria by a
factor of 10.
Culturing your dilution:
Once you have performed your dilutions, use need to culture the bacteria to see how many
there are. You can do this using the streak plate method for semi-quantitative results or by
using spread plates to give you an exact colony count.
Calculating the number of bacteria:
After your bacteria have grown, you can calculate the number of bacteria in your original
sample. To do this, simply multiply the number of bacteria on the plate by the dilution factor.
This tells you the number of bacteria in you NEAT sample.
For example, if you counted 56 bacterial colonies on your 1:1000 plate:
56 x 1000 = 56 000 cfu (colony forming units)
Remember to relate your result to the original sample. For example, if you swabbed a 10 cm
square, you found 56 000cfu/10cm 2. This is better shown as 5 600cfu/cm 2.