Comparison of Liquid & Solid
Bacterial Growth Media
LAB NOTES from Science Prof Online
Depending on the growth requirements of the bacteria and the information that the scientist hopes to gain, various types of growth media are available for culturing bacteria. Culturing means cultivating bacteria in a microbiology laboratory environment, and culture media contain the nutrients and other substances required to cultivate bacteria. There are several characteristics that any culture media must have.
Article Summary: Culture media basically come in solid & liquid form. Here are the main features of liquid nutrient broth versus solid agar used to cultivate bacteria.
Liquid vs. Solid Bacterial Growth Media
Portions of this article originally appeared on Suite101 online magazine.
Page last updated: 11/2012
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Basic Features of Bacterial Growth Media
Media must be prepared in such a way that it is sterile prior tobeing inoculated with a particular type of bacteria is cultured on that medium, it is the only type of bacteria present. Growth media must also provide everything the bacterial culture needs to live and grow.
Although there are a wide variety of different media formulations, all culture media can be divided into two very basic categories—liquid and solid.
Growing Bacterial Cultures in Liquid Nutrient Broth
Nutrient broth is a liquid bacterial growth medium made of powdered beef extract and short chains of amino acids that have been dissolved in water. Liquid medium is convenient to use for growing bacteria in test tubes, and can reveal information about the oxygen requirements of bacteria growing within. Bacteria that require oxygen will grow close to the water’s surface, and bacteria that cannot tolerate the presence of oxygen will grow at the bottom of the test tube.
Growing Bacterial Cultures on Solid Media
Sometimes solid, rather than liquid, growth media are required. Broth media can be made solid by adding agar, a gel like polysaccharide (big sugar) extracted from red algae. Broth with about 1.5% agar added will be liquid when heated, but solid at room temperature, making it easy to pour into a vessel, such as a Petri dish or test tube when hot. The solution then becomes solid once cooled.
When nutrient agar is poured into test tubes, it is often left to cool with the test tubes positioned at an angle, resulting in slant tubes or “slants.” Slants are handy in that they provide a large surface for aerobic (oxygen utilizing) bacteria to grow on, as well as an area of solid agar in which anaerobic bacteria (those that don’t use oxygen) can be grown.
Media that have been formulated to be solid at room temperature through the addition of agar, have the word “agar” in the name. Nutrient agar is the term for agar that has been combined with nutrient broth.
Bacteria cannot digest agar, and the broth-agar combination contains water, nutrients and often buffers to help regulate pH, all while providing a solid surface for bacteria to grow on. Bacteria growing on the surface of agar reveal their colony morphology - their characteristic appearance when growing in a group of bacteria that arose from one parent cell. Being able to view a species colony morphology can provide helpful, albeit general, information on the identity of unknown bacteria.
Sources & Resources
- Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.
- Perry, J. and Stanley, J. (1997) Microbiology Dynamics & Diversity. Saunders College Publishing.