As a researcher working with microorganisms, you likely have varying needs for microbial cultivation. When you grow microbes under controlled circumstances in the lab, you must provide a source of nutrients for their survival. These nutrients are contained in culture media. Many types of media are available, and which one you use depends on your intended purpose (Table 1). The kinds and amounts of nutrients can vary, and other ingredients can be added to promote selective growth of desired microorganisms.
Broth vs. Agar
Liquid (broth) and solid (agar) forms of culture media provide different growth environments for microbes, where the cells are either suspended in the broth or form colonies on the agar. Broth media are used for the planktonic (free-floating) phase of microbial growth, and agar media are used for the isolated colony or large biofilm phase of microbial growth. However, biofilm studies can be conducted in broth as well, by allowing the biofilms to grow on the surface. You will likely use both broth and agar for different phases of an experiment or research application. Typically, organisms are initially grown in broth to reach a particular growth phase, at which point the cells can be plated on agar for further studies or long-term storage. (Learn more about the history of agar as a microbiology tool.)
Rich vs. Minimal Media
In many cases, you can select a rich medium that supplies a diverse range of nutrients to the microbe. However, in some situations, such as for studying metabolism, you may need to control the nutrients available to the microbe, so you would use a minimal medium, which contains only the metabolites required for microbial growth (e.g., water, carbon source, and various salts that contain essential elements required for protein and nucleic acid synthesis).
Chemically Defined (Synthetic) vs. Complex Media
Culture media can be chemically defined (synthetic), with a known composition and quantity of nutrients; or complex, with an undefined assortment and quantity of nutrients. Chemically defined media are required for metabolic studies, where you may need to include or exclude certain metabolites. They are also useful for organisms that require specific nutrients for growth. Complex media, on the other hand, provide a variety of nutrients in unspecific quantities because they are composed of extracts and enzymatic digests of living organisms (e.g., peptones from animal, plant, or yeast). Therefore, they are suitable for most everyday needs for easy-to-grow microbes except for metabolic or physiological studies due to the undefined levels of the components.
Chemically defined media can be either rich or minimal, while complex media are usually rich. However, if you formulate your own media, you can add complex nutrients (e.g., yeast extract or tryptone) at minimal levels, but determining those levels would require growth studies.
Selective, Enriched, and Differential Media
By using the appropriate media, you can inhibit the growth of undesired microbes, promote the growth of microbes of interest, or distinguish colonies of different microbes. Selective Media allow the growth of a specified microbe because they contain ingredients that inhibit the growth of undesired microbes. Difficult-to-grow, or fastidious, organisms require even more nutrients than are provided in rich media. By using enriched media you provide the extra nutrients to help these organisms grow under laboratory conditions. Differential media contain ingredients that distinguish between different microbes and can indicate the presence of a certain microbe.
Table 1 summarizes the types of culture media described here. Microbe suppliers will provide growth media recommendations for their specific organisms, and additional information about growth media is available from ATTC.
Table 1. Glossary and examples of culture media.
|Broth||Liquid form||LB broth|
|Agar||Solid form||LB agar|
|Rich||Numerous nutrients||Tryptic soy broth|
|Minimal||Only nutrients essential for growth||M9 minimal medium|
|Chemically defined||Known assortment and quantity of nutrients||MOPS EZ Rich Defined Medium|
|Complex||Undefined assortment and quantity of nutrients||Brain heart infusion (BHI) broth|
|Differential||Ingredients that enable distinction between different microbes||MacConkey agar (MCK)|
|Selective||Ingredients that allow the growth of only a specific microbe||Mannitol salt agar (MSA)|
|Enriched||Extra nutrients to promote cultivation of difficult-to-grow (fastidious) organisms, such as Haemophilus influenzae and Neisseria spp||Blood agar|