Gram Negative Bacteria: Definition & Examples

Instructor: Artem Cheprasov
Have you ever wondered what Gram negative bacteria are or, at least, why they are called Gram negative in the first place? This lesson teaches you that and provides some famous examples of Gram negative bacteria.

What Are Gram Negative Bacteria?

Although they have the word 'negative' in their name, they're not all bad. Some have nothing to do with people, others are beneficial, and further others may be of harm to people. These are the Gram negative bacteria. Gram negative bacteria are so named because they are a group of bacteria that do not retain a special stain used in microbiology. You'll learn a bit more about why they're called Gram negative and then learn a few examples of them.

Why Are They Called Gram Negative?

Back in the late 19th century, a Danish scientist by the name of Hans Christian Gram developed a technique known today as the Gram stain. The technique essentially made bacteria more visible to scientists when examined under the microscope. A quick summary of the Gram stain that's pertinent to this lesson includes the use of:

  • A stain called crystal violet in order to stain the bacterial cells with a purplish (violet) color. This is used alongside an iodine solution.
  • A decolorizer, like ethyl alcohol
  • A counterstain, like safranin, which stains a pinkish-red color.

That's all you need to know about the Gram stain, for this lesson at least, in to understand why some bacteria are called Gram negative. So, let's say you have some cells on a slide. First you pour on the crystal violet and then the iodine solution. In short, this stains the cells on the slide purple.

Next, you use the decolorizer. And here's the important bit. Some cells, the ones that have a very thick cell wall called a peptidoglycan cell wall, will not decolorize. These cells, called Gram positive cells, will stay purple because this thick cell wall helps retain the crystal violet-iodine complex. However, cells with a thinner peptidoglycan cell wall, the Gram negative cells, will decolorize. Once those cells are decolorized, you use a counterstain to visualize them and with a different color than the Gram positive cells in order to differentiate between the two.

Example of Gram Negative Bacteria. Note the pinkish-red color.
Example of Gram Negative Bacteria

Examples of Gram Negative Bacteria

There are an awful lot of Gram negative bacteria. Since we can't possibly cover all of them, we'll just go over some really famous examples instead. Some of these names will probably ring a bell to you. Gram negative bacteria include:

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