Gram-Positive Bacteria: Definition & Examples

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  • 0:04 What Are Gram-Positive…
  • 0:34 Gram-Positive Classification
  • 2:15 Examples of…
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson is positively filled with information about gram-positive bacteria! First, you'll learn why these guys are even called gram positive in the first place. Then, you'll learn about a few examples of some famous gram-positive bacteria.

What Are Gram-Positive Bacteria?

If you're not positive you know some examples of gram-positive bacteria and what exactly makes them gram positive, you're about to find out! Gram-positive bacteria are those that can be stained violet using the Gram stain test. The rest are gram negative. That's the easy gist of it, of course. Let's find out a little bit more about this staining technique and why these cells are violet instead of some other cool color. Then let's look at some examples of famous gram-positive bacteria.

Gram-Positive Classification

To understand why gram-positive bacteria are so called, we need to travel back to more than 100 years ago, back to the late 1800s. Not only that, we need to book a ticket to Denmark to meet the Danish scientist Hans Christian Gram. He's the guy who developed the Gram stain. The Gram stain, simply put, uses the following ingredients:

  • A water-soluble stain called crystal violet
  • An iodine solution
  • A decolorizing agent such as ethyl alcohol or acetone
  • A counterstain, typically safranin

In order to stain a bunch of bacteria on a microscope slide, you'd first pour on the crystal violet dye. Then, you'd add iodine in order to form a crystal violet-iodine complex, which is a larger and water-insoluble molecule than the crystal violet alone. The iodine, in short, makes it more difficult for the crystal violet dye to be removed.

Of course, if all the cells are violet, how do we know which ones are gram positive or gram negative? Well, that's where the decolorizer comes in. After pouring on the decolorizer, only the cells with a thick wall called the peptidoglycan layer will retain the crystal violet-iodine complex and remain purple. Cells with a thin peptidoglycan layer, the gram-negative cells, will not. They'll lose their color instead.

Since it's kind of hard to see colorless cells, we need to stain the gram-negative cells with a counterstain like safranin. This turns the gram-negative cells red. The addition of this lighter counterstain doesn't affect the violet color of the gram-positive cells with their thick peptidoglycan cell walls. Thus, looking under the microscope, we see red gram-negative cells and purple gram-positive cells.

Examples of Gram-Positive Bacteria

There are many gram-positive bacteria, including the Staphylococcus genus (group of bacteria). These typically look like clusters of grapes underneath the microscope. One famous example found in this genus is Staphylococcus aureus. This bacterium is well known for MRSA, or methicillin-resistant Staphylococcus aureus. This is a kind of bacteria that's resistant to many antibiotics. As result, MRSA can cause serious skin infections, pneumonia, and blood infections.

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