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What Is a Colony-Forming Unit? - Definition & Purpose

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  • 0:04 Bacteria in Science
  • 0:41 Growing Bacteria
  • 3:02 Importance of CFUs
  • 4:03 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

Bridgett has a PhD in microbiology and immunology and teaches college biology.

Colony-forming unit describes how many bacteria were put onto a petri dish to grow into visible groups. How is the CFU used in science, and why is it important? Complete this lesson to find the answers.

Bacteria in Science

When you think of bacteria, you probably think of getting sick. When E. coli is in the news, it's about yet another outbreak of food poisoning. Scientists, however, consider E. coli and other bacteria as an extremely valuable, versatile tool.

Bacteria can be used to study genetics, immunity, proteins, infections, and molecular biology, to name a few. Bacteria are also used to make medicines and proteins for human use. One specific use of bacteria is the production of insulin. E. coli bacteria are engineered to produce this human protein, which is used by patients with diabetes to manage their blood sugar.

Growing Bacteria

Bacteria used for research are grown in petri dishes on agar, a gelatin-like substance sourced from seaweed, enriched with various nutrients. In laymen's terms, the bacteria live on a bed of food. Once they've been engineered to do whatever the scientist has been trying to make them do, they are spread out onto one of these agar-filled dishes.

Next, the bacteria are serially diluted. This means that the bacteria are diluted with growth medium, normally 1 part bacteria to 10 parts medium. This dilution is then diluted again, so it becomes 1:100. This happens until the bacteria are diluted up to 10,000-1,000,000-fold.

Performing serial dilutions doesn't just make more work for scientists; it also makes counting colonies on the petri dish easier. If there are too many colonies, they are indistinguishable. If there are too few colonies, the results might not be accurate. It's a classic Goldilocks problem. The best range for colonies on a plate is about 25-300. If there are less than 25 colonies, you should pick a different petri dish to count. If there are more than you can accurately count, you should also pick another dish. This is another advantage of serial dilutions: you can count colonies on more than one petri dish.

The diluted bacteria are plated onto the petri dishes and grown until visible colonies form. A colony is simply a group of bacteria that grew from one original bacterium. The colonies are then counted since you can't see individual bacteria without a microscope. Rather than saying the number of colonies on the agar equals the number of bacteria originally plated, scientists talk about the number of colony forming units (a CFU). A colony forming unit is normally one bacterium or a small group of bacteria that were able to replicate many times to form one single, visible colony.

The term CFU is used because it is impossible to be certain that each colony came from only one bacterium. Also, counting CFUs instead of the total bacteria in a sample lets the scientist ignore dead bacteria or bacteria that cannot replicate under the conditions being tested (such as temperature, growth medium, humidity, and oxygen content). Because of these reasons, CFU is the best estimate of the number of bacteria that will actively do whatever job the scientist is trying to get them to do.

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