Gram Negative Bacteria

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What Does Gram Negative Mean?

Many types of bacteria can be divided into two groups: Gram negative and Gram positive. In order to determine which category a particular bacteria falls into, the bacteria must undergo the Gram staining procedure. The sample is stained twice - first with crystal violet, then a decolorant is added, and then a second dye is added. At the end of the process, most bacteria will be stained either purple (meaning they are Gram positive) or pinkish-red (meaning they are Gram negative).

The staining process can differentiate between the two types of cells due to the presence of a thick peptidoglycan layer in Gram positive bacteria. This thick layer retains the crystal violet dye, while the dye is easily washed off of the single layer of peptidoglycan in Gram negative bacteria. The thin layer retains the pink or red color from the second dye.

Other characteristics of Gram negative bacteria include a periplasmic space, an outer membrane, and high lipid, lipoprotein, and lipopolysaccharide content. If the bacteria produce a toxin, it is typically an endotoxin.

Examples of Gram Negative Bacteria

There are many species of Gram negative bacteria, so it is difficult to make generalizations about them. Some are commonly found strains, other are rare. Some are commensal, meaning they live on or in our bodies without causing harm. Others are pathogenic, meaning they are capable of causing disease. Some species of Gram-negative bacteria can be harmful or benign, depending on the strain. Common examples of Gram negative bacteria include:

Escherichia coli: Commonly known as simply E. coli, these bacteria are found in the intestines of humans and almost all other warm-blooded organisms. Many strains are just a normal member of the average gut bacteria. Some strains, however, like E. coli O157:H7, cause serious foodborne illness.

Campylobacter jejuni: This Gram negative bacteria is one of the leading causes of food poisoning. It is an opportunistic pathogen, meaning it usually lives in humans without causing disease but can cause illness once the individual’s immune system is weakened.

Bordetella pertussis: This bacteria causes pertussis, commonly known as whooping cough. Fortunately there is a vaccine available to provide protection against this Gram negative microorganism.

Salmonella: This genus is closely related to E. coli, and it is also often found in humans without causing illness. Many strains are pathogenic, and Salmonella bacteria are responsible for thousands of cases of food poisoning each year. This bacteria can also cause typhoid fever.

Other well-known Gram negative bacteria include the causative agents of Legionnaires’ disease, gonorrhea and cholera.

The Future of Gram Negative Bacteria

Recently, Gram negative bacteria have been mentioned in the news in relation to outbreaks that are usually hospital-acquired infections. Some Gram negative bacteria have been labeled “superbugs,” due to the fact that they are often resistant to antibiotics, spread very easily and can lead to dangerous infections. It is likely that we will continue to see Gram negative bacteria in the news for years to come.

It is important to remember that not all Gram negative bacteria are “bad” or pathogenic. We encounter these microorganisms everyday, and many live in or on our bodies.


Bruckner, Monica Z. “Gram Staining.” Microbial Life Educational Resources.

CNN Health. “Gram-Negative Bacteria are Drug-Resistant Superbugs to Watch Out For.” February 20, 2009.

Rollins, D.M. and S.W. Joseph. 2000. “Comparative Characteristics of Gram-Positive and Gram-Negative Bacteria.”

Todar, Kenneth. Todar’s Online Textbook of Bacteriology. 2011.