Use of Liquid Nutrient Broth Media for Growing Bacteria

Dec 10, 2010
Tami Port

Sedimented Bacterial Growth Swirled in Test Tube - T. Port

Broth is a nutrient-infused liquid medium used for growing bacteria. Here is a summary of the advantages and disadvantages of this type of growth medium.

In order for scientists to experiment with, and learn more about microbes, investigators must be able to grow these tiny organisms in a laboratory by providing them with water, food and an environment to grow in. Media are initially sterilized, then inoculated with the bacteria that is of interest and finally incubated for twenty four hours or more, to encourage bacterial growth.

Broth and Agar Bacterial Growth Media

The two main types of bacterial growth media used are liquid broth and solid, Jell-o-like agar. Each has specific advantages and disadvantages. The growing environment used will depend on what the researcher wants to do with, or learn from, the microbes.

Nutrient Broth Bacterial Growth Medium

Nutrient broth is typically made of a powdered beef extract that contains peptones (broken down proteins). The powder is dissolved in water, put in test tubes, and sterilized.

Broth is convenient, as most bacterial samples can be easily introduced into and grown in this type of medium, even those with widely different aerotolerance (oxygen requirements).

Different Types of Oxygen Requirements of Bacteria

Unlike animals, bacteria do not all require oxygen. Some types of bacteria are poisoned by oxygen, others can take it or leave it. Liquid broth allows bacteria to grow at varying oxygen levels, which decrease as the depth of the broth increases. See the test tube diagram at the end of this article for an illustration of the aerotolerance dependent growth patterns of microbes:

Obligate aerobic bacteria, those that must have oxygen to extract energy from food, will gather at the top of the test tube, near the broth’s surface, to absorb maximum amount of oxygen (1).
Obligate anaerobic bacteria, those killed by oxygen, will gather at the bottom of the test tube, staying as far away from oxygen as possible (2).
Facultative bacteria, those that can live with or without oxygen, will gather mostly at the top, since aerobic respiration is the most energy efficient way to turn food into energy; but since lack of oxygen does not hurt these microbes, they can survive anywhere in the broth (3).
Microaerophiles gather at the upper part of the test tube but not at the top. These microbes require oxygen, but at concentrations lower than those found in the atmosphere (4).
Aerotolerant bacteria are not affected at all by oxygen, so they can be found evenly spread along the test tube (5).

Other Factors Influencing Bacterial Growth Patterns in Broth

In addition to oxygen requirements, there are other factors that can influence microbial growth patterns in liquid. Motile bacteria (those with flagella) can swim. Their movement will create a uniform cloudiness (turbidity) in the broth. Non-motile bacteria with waxy cell walls tend to float at the surface of the broth, producing a surface membrane called a pellicle. Other types of non-motile bacteria sink to the bottom of the tube, forming sediment; some of these bacteria tend to stick together in clumps called flocculent growth.

Disadvantages of Using a Broth Bacterial Growth Medium

Although there are many advantages to growing bacteria in broth, one of the main disadvantages is that bacterial colonies do not form in a liquid suspension. A colony is a collection of bacteria that have arisen from the division of a single parent bacterium, meaning that all bacteria in a colony are the same type. Being able to view colonies can help researchers see if a growth medium has been contaminated with another type of unwanted bacteria, since, in some cases, the colony of contaminant bacteria will look different that the majority of colonies present. When bacteria grow dispersed in liquid, colony morphology cannot be assessed.

For more information on microbiology, see the SPO Virtual Microbiology Classroom or Todar's Online Textbook of Bacteriology.