Back To CourseMicrobiology 101: Intro to Microbiology
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
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.
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.
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.
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.
At this point, you should have a good handle on why microbiologists would use selective and differential media. But in the intro scenario, we used peanuts as both a differential and a selective agent. Today, many media recipes have been developed to be both selective and differential. Ingredients have been included that inhibit the growth of certain bacterial species while also distinguishing between the different species that are able to grow. The combination eliminates the need for culturing twice: once to select for a specific group and the second time to differentiate between members of that group.
The MacConkey agar we talked about as an example of a selective media is also a differential media. In addition to the selective bile salts and crystal violet, MacConkey agar contains a pH indicator and the sugar lactose. Some Gram-negative bacteria are able to ferment lactose, producing an acidic byproduct. The pH indicator makes these colonies appear red. Non-lactose fermenters will be colorless. So, the MacConkey agar selects for Gram-negative bacteria and differentiates between lactose fermenters and non-fermenters.
There are countless examples of selective and differential media. The two presented here are just the tip of the iceberg. Some select by including very specific energy sources or antibiotics in the media. Others are able to differentiate through gas production, metabolic waste production or the presence of genetically modified DNA. The possibilities are endless.
Let's review. In order to study bacteria, microbiologists need to grow the cells in the lab; this is called culturing. Sometimes a bacterial sample is mixed, meaning it contains multiple species of bacteria, and sometimes only one or a few species are of interest. In order to only culture the desired bacteria, a scientist can change the components of the media used to grow the bacteria.
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. MacConkey agar is a selective medium containing bile salts and crystal violet to inhibit the growth of Gram-positive bacteria while allowing for the growth of Gram-negative species.
A differential media contains specific ingredients or chemicals that allow the observer to visually distinguish which species possess and which species lack a specific biochemical process. Blood agar is a differential medium that distinguishes bacterial species by their ability to break down red blood cells.
The ability to break down the cells will cause a change in the color of the blood agar. Some media are both differential and selective. Combining the two can often eliminate having to perform multiple cultures, saving time and money for the microbiologist. MacConkey agar can both select for Gram-negative bacteria and differentiate lactose fermenting and non-fermenting bacteria.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons