Selective Media: Videos & Practice Problems

Selective media is designed to promote the growth of specific microorganisms while inhibiting others. A prime example is MacConkey agar, which isolates gram-negative intestinal bacteria by using crystal violet to inhibit gram-positive bacteria and bile salts to suppress non-intestinal species. This selective environment allows for the isolation of desired bacteria from mixed cultures, demonstrating the importance of selective media in microbiology for studying specific microbial populations.

Selective Media

In this video, we're going to begin our lesson on selective media. Selective media is a specific type of microbial growth media designed to promote only the growth of the species of interest and to inhibit the growth of other species that are not of interest. A classic example of selective media is MacConkey agar, a specific type of selective media commonly used in the medical field to isolate gram-negative intestinal bacteria. MacConkey agar contains crystal violet, a specific dye that inhibits gram-positive cell growth, therefore promoting gram-negative cell growth. It also contains bile salts, which inhibit non-intestinal bacteria, helping to promote intestinal bacterial growth. Between the crystal violet and the bile salts, they promote or select for the growth of gram-negative intestinal bacteria.

If we take a look at this image below, we can see how using MacConkey agar can help isolate gram-negative bacteria that grow in the intestine. Notice on the left-hand side over here, what we have is a tube containing a liquid broth of a mixture of bacterial cells, a mixed culture of bacterial cells including gram-positive gut bacteria. We can take this mixture of bacterial cells and transfer some of it to this MacConkey agar plate. Because the MacConkey agar plate contains crystal violet and bile salts, it will inhibit the growth of gram-positive cells and non-intestinal bacteria. It is helping to select for gram-negative intestinal bacteria. What you can see here on the MacConkey agar plate is that the only colonies that form are colonies of intestinal gram-negative bacteria. All other cells are going to be inhibited, and so they will not appear or grow on the MacConkey agar plate. The MacConkey agar plate is a type of selective media because it is selecting for a very specific species of interest to grow while inhibiting the growth of other species that are not of interest.

This here concludes our brief lesson on selective media, and we'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.

A type of culture medium that induces the growth of one particular organism in a mixed population of cells is:

• A. Synthetic medium.
• B. Specific culture medium.
• C. Complex medium.
• D. Selective medium.

Answer: (D)

Here’s what students ask on this topic:

What is selective media in microbiology?

Selective media in microbiology is a type of growth medium designed to promote the growth of specific microorganisms while inhibiting the growth of others. This is achieved by incorporating specific substances that favor the growth of the desired microorganisms and suppress the growth of unwanted ones. For example, MacConkey agar is a selective medium that isolates gram-negative intestinal bacteria by using crystal violet to inhibit gram-positive bacteria and bile salts to suppress non-intestinal species. This selective environment is crucial for isolating and studying specific microbial populations from mixed cultures.

How does MacConkey agar work as a selective medium?

MacConkey agar works as a selective medium by incorporating substances that inhibit the growth of certain bacteria while promoting the growth of others. It contains crystal violet, which inhibits the growth of gram-positive bacteria, and bile salts, which suppress non-intestinal bacteria. This combination allows for the selective growth of gram-negative intestinal bacteria. When a mixed culture is plated on MacConkey agar, only the gram-negative intestinal bacteria will form colonies, while other bacteria are inhibited. This makes MacConkey agar an effective tool for isolating and studying specific bacterial populations.

Why is selective media important in microbiology?

Selective media is important in microbiology because it allows researchers to isolate and study specific microorganisms from mixed cultures. By promoting the growth of the desired microorganisms and inhibiting others, selective media helps in identifying and characterizing specific bacterial populations. This is particularly useful in medical microbiology for diagnosing infections, in environmental microbiology for studying microbial communities, and in industrial microbiology for selecting beneficial strains. Selective media thus plays a crucial role in advancing our understanding of microbial diversity and function.

What are some examples of selective media besides MacConkey agar?

Besides MacConkey agar, there are several other examples of selective media used in microbiology. Mannitol Salt Agar (MSA) is used to isolate Staphylococcus species by containing high salt concentrations that inhibit most other bacteria. Eosin Methylene Blue (EMB) agar is used to isolate gram-negative bacteria, particularly coliforms, by incorporating dyes that inhibit gram-positive bacteria. Hektoen Enteric (HE) agar is used to isolate Salmonella and Shigella species by containing bile salts and dyes that inhibit gram-positive bacteria and other non-enteric organisms. These examples highlight the diverse applications of selective media in isolating specific microbial populations.

How do bile salts in MacConkey agar inhibit non-intestinal bacteria?

Bile salts in MacConkey agar inhibit non-intestinal bacteria by disrupting their cell membranes and interfering with their metabolic processes. Non-intestinal bacteria are generally not adapted to the presence of bile salts, which are naturally found in the intestinal environment. The bile salts create a hostile environment for these bacteria, preventing their growth. In contrast, intestinal bacteria have evolved mechanisms to tolerate bile salts, allowing them to thrive on MacConkey agar. This selective inhibition is crucial for isolating gram-negative intestinal bacteria from mixed cultures.