CFU Calculation Formula
When trying to find the right CFU calculation formula, it helps to first understand the nature of CFUs. We also need to discuss the process around growing them in order to count them and perform calculations. Let’s begin at the beginning.
What Is a Colony Forming Unit (CFU)?
When we work on growing bacteria in a lab either to see how they perform or how they react, we need to measure the growth or inhibition of growth of those bacteria.
To do this important work, scientists will collect bacteria or other organisms on a growth medium like agar inside of a Petri dish and allow the bacteria to develop.
Over time, scientists can observe the growth of the bacteria by watching how it spreads.
Where once the spreading bacteria was simply considered, and called, a colony, we now refer to the growing bacteria as colony forming units, or CFUs, because we cannot be certain that all of the bacteria in a particular area belong to the same colony.
In general, bacteria will start out small and start to spread outward in what resembles a circle.
Also read: What are Colony Forming Units
How to Grow Colony Forming Units
To grow CFUs, either to monitor how the bacteria grows or to test whether a particular antibiotic is effective on the bacteria, scientists collect a sample of the bacteria, typically in a vial.
Then, they will open a Petri dish with a growth medium on it, usually agar, often with some sort of nutrient specific to the growth of the bacteria they are working with.
Then, they will swap a sample of the bacteria from the vial onto a cotton swab. The cotton swab will then be swiped across the agar several times in multiple rotations to ensure the entire surface is covered in the bacteria.
Then, within 24 hours, scientists are able to count the number of CFUs growing on the plate by observing the bottom of the Petri dish and marking each CFU visible to the naked eye with a black marker. That approach is the basic answer to “how to count colony forming units.”
But the process is not always basic. The problem many scientists encounter when this procedure is performed is that sometimes the CFUs are numerous. When there are too many CFUs in a single sample, it can be impossible to differentiate one from the other.
When we cannot count the CFUs, we cannot accurately tell if or how they are growing.
To overcome this problem, we have to dilute the sample, and this is where the CFU calculation formula comes into play.
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CFU Calculation Formula
To dilute the bacteria sample and effectively count the CFUs, you will need Pasteur pipettes, sterile water, and vials with stoppers. Be sure also that you are in a sterile room, using sterile gloves, and minimizing the opportunity for new bacteria to be introduced into your samples.
To begin, you will take 1 ml of the bacterial sample and put it into a clean vial. Next, add 9 ml of sterile water to the vial. Gently shake the vial with the stopper on to thoroughly combine the ingredients. This is a 1:10 dilution. Depending on how dense your sample is, you may need to dilute the sample one or two more times.
You will know you need to dilute again if you try to count your CFUs on agar and the spread is too closely connected that you cannot differentiate.
Keep track of how many times you dilute the sample.
Then, when you want to figure out how many CFUs are in the undiluted sample, you just have to scale back up.
For example, if you dilute the sample at a 1:10 ratio 5 times, you have the figure 10^5. You then simply take that figure and multiply it by the number of CFUs you count in the diluted sample.
So, if you count 13 CFUs on your plate, your CFU calculation formula would be:
13 x 10^5 = 1,300,000
Thus, you can assume you have about 1,300,000 colony forming units in your original, undiluted sample.
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Summing It All Up
In the end, the CFU calculation formula is simple as long as you keep track of how many times you have diluted the sample. As with any mathematical calculation, when you first divide, you simply multiply to get back to where you started.
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