Then, you take 1 mL of the 1:10 dilution and add that to 9 mL of liquid media. Since the dilution is one part sample to nine parts diluent, it is referred to as a 1:10 dilution. This means you should further dilute your sample and try growing again so that you can see individual colonies.Įxperimental Procedure for Finding CFU An example of using dilutions to do a CFU calculation could go as follows: First, from the initial sample your teacher gives you, you take 1 mL and plate it.įirst, you take 1 mL of sample and dilute it into 9 mL of liquid media. If the sample was too concentrated then instead of individual colonies you will see a large area covered with bacterial growth which is called a lawn. Multiple dilutions are often a good idea since the original sample can be very concentrated.Īfter allowing the bacteria to grow on the plates for a given amount of time, individual colonies are counted on a plate. The dilutions are then plated onto plates with the correct growth medium. Using Dilutions to Find CFU The procedure to find CFU of a given sample involves first diluting that sample. You can try and take a small part of your original sample, figure out how many bacteria there are in that part, and then calculate to approximate out how many there might have been in the original sample.
So how can you figure out how many bacteria you have In the same way that you can take a poll of 1,000 people and apply it to 10,000 people to figure out how they might think about a certain subject. You are only interested in viable bacteria, or bacteria that can replicate. Furthermore, some of them may be dead and others alive, and the dead ones should not count toward the total.
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