Differential & Selective Media in Microbiology

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  • 0:07 Selective and…
  • 1:20 Culture Media
  • 2:18 Differential Media
  • 3:35 Selective Media
  • 4:23 Selective and…
  • 5:50 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Every microbiologist eventually has to grow cultures of bacteria for their experiments. In this lesson, we will investigate selective and differential media, two techniques to make culturing bacterial species a little more efficient.

Selective and Differential Foods?

I bet that every one of us knows someone who is allergic to peanuts. Standing in a crowded room, it would be impossible to identify the individuals with this allergy simply by looking at them - assuming no one is wearing a medical alert bracelet. But what if we really wanted to know who was allergic to peanuts and who was not? What could we do?

One option, and I'm not suggesting you try this at home, would be to give everyone in the room a cookie laced with peanuts. Very quickly, the allergic people in the room would start showing symptoms. They might start having trouble breathing, swell up or start vomiting. In any case, it would be very easy to differentiate the mixed population into allergic and non-allergic groups. The peanuts would serve as a differential food.

Let's take this a little further. Now that you know who is allergic to peanuts, suppose you want to encourage the survival of peanut-resistant people while inhibiting the survival of the peanut-allergic. All you would have to do is provide these people only peanut-based foods. People that could eat peanuts would be fine. People that could not would either starve or die of an allergic reaction. You have just selected for the peanut-resistant people. Here, the peanuts would serve as a selective food.

Culture Media

These experiments might seem a bit cruel and out of place in the human world, but they are occurring in the bacterial world daily in microbiology labs, hospitals and classrooms. Learning how to grow and manipulate bacteria is a very useful skill for a microbiologist. Let's quickly review a few key concepts before we get to the meat of this lesson. Bacterial culturing is the process of growing bacteria in a medium. A culture can contain only one species of bacteria, called a pure culture, or can contain many different species, called a mixed culture.

A medium is a liquid or gel designed to support the growth of a bacterial culture. The medium has to contain everything that the bacteria needs for growth and survival. There are countless media variations depending on what bacterial species you are attempting to grow. And, just like the peanuts in the introduction, you can even include ingredients to differentiate one species from another or select for the growth of a certain type of bacteria while inhibiting others.

Differential Media

The first type of media we'll investigate is differential media. A differential media contains specific ingredients or chemicals that help us to visually distinguish which species do or don't carry out a specific biochemical process. To clarify this definition, let's look at an example of a differential medium.

Blood agar is a differential medium that distinguishes bacterial species by their ability to break down the red blood cells included in the media. Blood agar is often used to distinguish between the different species of pathogenic Streptococcus bacteria. The different types of Strep each have a predictable pattern of hemolysis, which is simply the breakdown of red blood cells. Alpha-hemolytic Streptococcus, like those that cause pneumonia, produce a narrow band of slimy discoloration around the colony during the partial breakdown of the red blood cells.

Beta-hemolytic Streptococcus, like those that cause potentially deadly infections in newborns, produce a completely clear zone around the colony during the complete breakdown of the red blood cells. Gamma-hemolytic Enterococcus, which used to be called Group D Streptococcus, don't change the media appearance at all since they can't break down the red blood cells. In these pictures, you can see how simply growing these bacteria on blood agar can help determine what type of Streptococcus you're dealing with.

Selective Media

The next type of media is selective media. A selective media is composed of specific ingredients to inhibit the growth of certain species of microbes in a mixed culture while allowing others to grow. Again, in order to fully grasp this definition, let's look at an example of a selective medium.

MacConkey agar is a selective medium containing bile salts and crystal violet that inhibits the growth of Gram-positive bacteria while allowing for the growth of Gram-negative bacteria. MacConkey agar is often used when investigating and culturing fecal bacteria. Many disease-causing bacteria found in human feces are Gram-negative, like E. coli, a common cause of food poisoning. Gram-positive species are unable to grow in the presence of bile salts and crystal violet, ensuring that more colonies will be the Gram-negative pathogens of interest.

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