1.3.3: Types of Media – Functional/Application Classifications

Selective Media -

Selective media are media that contain ingredients that act to encourage the growth of some types of bacteria while inhibiting the growth of other types of bacteria. This characteristic of the media allows the microbiologist to isolate specific types of organisms on the media. Examples of selective media include mannitol salt agar media, MacConkey agar media, and eosin methylene blue agar.

Differential Media -

Differential media contain ingredients that cause distinct types of bacteria to grow differently on the same media. Those differences may include changes in the color of the media, changes in the color of the bacterial colonies, or other visible reactions. Blood agar is one type of differential media that allows bacteria to be differentiated based on if and how blood cells are processed by the bacteria growing on the media. Some media are both selective and differential, such as mannitol salt agar, MacConkey agar, and eosin methylene blue agar.

Figure 1. Mannitol salt agar. Source: Ajay Kumar Chaurasiya, Wikimedia Commons, CC BY 4.0.

Figure 2. MacConkey agar. Credit: Ajay Kumar Chaurasiya, Wikimedia Commons, CC BY-SA 4.0.

Selective and differential media are especially useful in clinical microbiology because they allow microbiologists to encourage the growth of certain organisms while also observing visible reactions that may help distinguish one group of bacteria from another.

Enriched Media -

Enriched media are used to grow fastidious microorganisms, which are microorganisms requiring additional nutrients and growth factors, such as amino acids, lipids, or other specialized nutrients. Enriched media contain special ingredients such as blood or serum. Examples of enriched media include blood agar and chocolate agar.

Anaerobic Media -

Anaerobic media are a specialized type of media used to grow bacteria that require low oxygen or the absence of oxygen. These bacteria are called anaerobes. Anaerobic media are made using an ingredient, such as sodium thioglycolate, that binds to and reduces oxygen in the media, thereby creating an environment favorable for anaerobic bacterial growth.

Transport Media -

Transport media are media specialized for the clinical setting. Transport media contain ingredients such as buffers and salts that serve to maintain the microorganism being transported without permitting its multiplication. There are a variety of transport media designed for the different types of microorganisms that must be identified in clinical laboratories.

Your instructor will have several examples of media of different physical characteristics available for your observation. You should observe each of the examples.