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What are Gram-Negative Bacteria?

Mary McMahon
Updated Jun 04, 2024
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Gram-negative bacteria are bacteria which do not turn purple in the Gram staining process used as a basic step in the identification of bacteria. Most bacteria can be divided into either Gram-positive or Gram-negative bacteria, reflecting key differences in the composition of their cell walls. These differences often have a direct influence on what the bacteria does, with some Gram-negative bacteria being pathogenic in nature.

The Gram stain was developed in 1884 by Hans Christian Gram. In this process, bacteria is fixed on a slide and then bathed in crystal violet, the primary staining solution. All of the cells on the slide turn purple, after which a mordant such as iodine is added to fix the color. Then, a decolorizer is added to the slide. If the bacteria is Gram-negative, the decolorizer will wash the crystal violet away, because the permeable cell wall does not allow the crystal violet to stain the bacteria. Then, a secondary stain is added, turning Gram-negative bacteria a pale pink, but having no effect on the already purple Gram-positive bacteria.

These bacteria have thin cell walls with an outer layer composed of proteins and lypopolysaccharide. This outer layer sometimes reacts with the immune system, causing inflammation and infection. In addition to preventing the bacteria from staining, the outer membrane of the cell also helps the bacteria resist an assortment of drugs, making treatment of infections with Gram-negative bacteria rather challenging.

Some examples of Gram-negative bacteria include Legionella, Salmonella, and E. Coli. Numerous other pathogens are also Gram-negative, including some forms of meningitis, a number of bacterial sources of gastrointestinal distress, and spirochetes. Gram-negative bacteria can be stubborn infectious agents, and many sources of lethal infection are Gram-negative, including the bacteria which contribute to secondary infections in hospitals and clinics.

Gram staining can provide insight into the composition of a bacterium's cell wall, so it is a routine step in examining new bacteria in the laboratory. Once bacteria has been subjected to a Gram stain, additional research will be needed to identify the bacteria, the source, and how infections caused by the bacteria might be treated, but the Gram stain provides a good first step. The stain also has the added benefit of highlighting the key structures of bacteria, including the inner structures of the cell, making them easier to see and understand. Gram staining doesn't work on all bacteria, however; Gram-indeterminate and Gram-variable bacteria cannot be identified this way.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All Things Nature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By pharmchick78 — On Oct 05, 2010

@earlyforest -- I assume you want to know more about the physical characteristics of a gram negative bacillus? Bacteria species vary so much that there's not really one behavioral characteristic of gram negative bacteria, so I'll stick with the physical.

The main difference between gram negative or positive bacteria is the structure of their cell wall. Gram negative bacteria have more layers to their cell walls than gram positive bacteria, which can protect the bacillus from some antibiotics, detergents, or dyes, and can also act an an endotoxin, which is what causes the body to get sick.

Although this is a very basic overview, that is the main difference between gram positive and gram negative bacteria, in layman's terms -- hope that helps you!

By EarlyForest — On Oct 05, 2010

What are some of the characteristics of gram stain negative bacteria? I know that they turn pink on a gram stain, but other than that, I don't really know the difference between gram negative/positive bacteria.

Can you tell me a little more about gram negative bacteria characteristics?

By LittleMan — On Oct 05, 2010

I wish it was so easy to identify a bacteria, but unfortunately gram staining is often just the first step to identifying a bacteria. For instance. For instance, in the case of something like an aerobic gram negative bacteria, you might have to run further tests for extended gram negative bacteria identification.

However, sometimes it does work out to where you can tell a bacillus just from its gram negative bacteria/bacillus stain.

I am by no means discounting the usefulness of a gram stain, but just bear in mind that gram negative bacteria classification can sometimes be more in-depth.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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