by Leslie Kent
Fun Rating: 4/5
Difficulty Rating: 2/5
What is the general purpose?
Microorganisms, like bacteria, are living things so small we cannot see them with our eyes and need to use a microscope. For some science experiments, such as determining how well an antibiotic kills a population of bacteria, it is important to be able to quantify how many living bacteria are present in a sample. Calculating colony forming units (CFUs) is a very common way to accomplish this.
Why do we use it?
Because they are so tiny, many bacteria can grow in a really small amount of space. For example, it is estimated that there are one billion (1,000,000,000!) bacteria living in just 1 milliliter (equal to less than 1 teaspoon) of your saliva. It would not be possible for us to count that many bacteria by hand every time we do an experiment, so we need a faster way to figure out the number of bacterial cells in a sample. There are multiple ways to compare how many bacteria are in a liquid, like optical density, but this method cannot distinguish between cells that are viable (alive) or dead. Because colonies are only formed from living bacterial cells, calculating colony forming units allows us to determine how many living bacteria cells are present.
How does it work?
A colony forming unit (CFU) is a visible group of cells that originate from one mother cell. The mother cells are too small to see without a microscope. But, if we put food for bacteria into a plate called a petri dish, and spread a small amount of bacteria on these plates to grow, the bacteria will replicate until they produce a colony big enough for us to see with just our eyes.
Colonies of Klebsiella pneumoniae growing on a petri dish. An individual colony is circled in purple. Pen included for scale. Image taken by Leslie Kent
If you have liquid containing 8 bacterial cells total, and you spread that liquid onto a food-containing petri dish, you can expect to see 8 colonies after the bacteria grow:
Image created by Leslie Kent
But, if you have liquid containing 1,000 bacterial cells, and you spread that liquid onto a food-containing petri dish, you would not be able to count colonies because they would overlap with each other:
Image created by Leslie Kent
The solution to this problem is to dilute the liquid so that there is a reasonable number of bacteria to count on the petri dish. Then, we can use math to figure out how many bacteria were in the original sample. For example, if we dilute the liquid 1:100 (1 part bacteria-containing liquid for every 99 parts of sterile liquid), and spread that diluted liquid onto the petri dish, we would expect to count approximately 10 colonies. If we multiply the 10 colonies we counted by 100 because we did a 1:100 dilution, we can determine that there were 1,000 bacterial cells in the original liquid. This equation can be written as:
# of bacterial cells total = (# of CFUs counted)(dilution factor)
Can you calculate how many bacteria were in the original liquid in the example below?
Image created by Leslie Kent
Answer:
(12 CFUs counted) x (1,000) = 12,000 bacterial cells
