Broth Culture: Definition, Medium & Characteristics

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  • 0:04 What Is Broth Culture?
  • 0:42 Why Are Broth Cultures Used?
  • 1:26 Broth Culture Bacteria…
  • 2:13 Broth Culture Medium
  • 3:35 Lesson Summary
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Lesson Transcript
Instructor: Bridgett Payseur

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

Broth cultures are used by scientists to grow bacteria. Learn why broth cultures are used, some characteristics of bacteria in broth cultures, and the different types of media available.

What Is Broth Culture?

What do you think of when you hear the word broth? Probably a flavorful liquid made from chicken, beef, or veggies and used as a base for soups, right? Bacteria and other microbes, like yeast, can be cultured in broth. It's not the same broth you'd find in your soup, but it has similarities.

Broth cultures are liquid cultures used to grow bacteria in laboratories. To create a broth culture, a scientist begins with a sterile liquid growth medium. The medium is inoculated with bacteria and placed in an incubator at the appropriate temperature. After a certain amount of time has passed, the broth becomes cloudy from the increased number of microbes.

Why Are Broth Cultures Used?

Why would a scientist choose to use a broth culture over agar? Broth cultures are convenient for growing a large number of bacteria very quickly. Scientists use these bacteria for other tests, combine them with freeze-medium for long-term storage, or inoculate them into other media for further experimentation.

Broth cultures are basically a way for scientists to grow and maintain their cultures of bacteria. They're also useful for bacteria that have been in a stressful experiment. It's hard to think of bacteria being stressed, but some experiments involve subjecting bacteria to harsh conditions like high heat. After the experiment, the bacteria can recover in a highly nutritious broth culture. It's a similar concept to an athlete going in a hot tub after a grueling football game.

Broth Culture Bacteria Appearance

You might think that bacteria in a broth culture are suspended equally throughout, like a well-blended soup. However, different bacteria behave differently. Some bacteria may remain dispersed evenly; others may clump together at the bottom of the culture or form a layer on top of the broth. Mostly, the bacteria's location has to do with whether or not the bacteria need oxygen. There are three different types:

  1. Aerotolerant anaerobic bacteria are evenly dispersed throughout the culture. While they don't require it, oxygen isn't poisonous to them, and they can live anywhere in the culture.
  2. Obligate anaerobic bacteria can't survive in the presence of oxygen, so they clump together at the bottom of the culture.
  3. Aerobic bacteria, which require oxygen to live, form a layer at the top of the medium. This allows them to be as close as possible to the oxygen source.

Broth Culture Medium

The growth medium is the liquid that the bacteria is grown in. Luria broth (LB broth), also called lysogeny broth or Luria-Bertani broth, is among the most common media for broth cultures. It was developed in the 1950s, primarily for growing Eschericia coli bacteria. It's used to help allow the bacteria to grow. LB broth is an undefined medium. Growth media can be defined by several other categories:

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