Difference Wiki

Gram-positive vs. Gram-negative: What's the Difference?

Edited by Janet White || By Harlon Moss || Updated on October 22, 2023
Gram-positive bacteria have a thick peptidoglycan layer and stain purple, while gram-negative bacteria have a thin layer and stain pink.

Key Differences

Gram-positive and gram-negative refer to two distinct classifications of bacteria based on their cell wall composition and reaction to the Gram stain procedure. Gram-positive bacteria exhibit a characteristic thick peptidoglycan layer in their cell walls, which retains the violet stain when subjected to this procedure. On the other hand, gram-negative bacteria, due to their thin peptidoglycan layer sandwiched between two membranes, do not retain the violet stain and take on a pinkish color instead.
Both gram-positive and gram-negative classifications have essential implications in the realm of medicine. The structural differences in their cell walls not only influence staining properties but also their susceptibility to antibiotics. Gram-positive bacteria, because of their single thick outer layer, are often more susceptible to antibiotics than gram-negative ones. The latter possesses an outer lipid membrane that can act as a barrier to many drugs.
When considering disease implications, both gram-positive and gram-negative bacteria are associated with human infections. However, the diseases they cause and their overall impacts can vary significantly. For instance, common gram-positive infections include strep throat caused by Streptococcus or skin infections by Staphylococcus. Conversely, gram-negative bacteria, such as E. coli, can cause gastrointestinal diseases or, in the case of Salmonella, food poisoning.
In the broader scope of biology, understanding the distinction between gram-positive and gram-negative bacteria helps researchers and clinicians in various ways. For researchers, this distinction aids in bacterial identification, taxonomy, and understanding evolutionary relationships. Clinically, knowing whether bacteria are gram-positive or gram-negative can guide treatment choices, ensuring effective medical intervention.

Comparison Chart

Peptidoglycan Layer Thickness


Color After Gram Staining


Outer Membrane


Susceptibility to Antibiotics

Generally more susceptible
Often less susceptible due to outer membrane

Examples of Diseases

Strep throat, Skin infections
Gastrointestinal diseases, Food poisoning

Gram-positive and Gram-negative Definitions


Bacteria with a thick peptidoglycan cell wall.
Staphylococcus is a type of gram-positive bacteria.


Bacteria with a thin peptidoglycan layer between two lipid membranes.
E. coli is an example of gram-negative bacteria.


Bacterial classification based on cell wall structure.
Doctors determined the infection was caused by a gram-positive strain.


Bacteria that turn pink in Gram staining due to not retaining violet stain.
The lab results indicated a high concentration of gram-negative bacteria.


Bacteria usually appearing purple under a microscope after staining.
The slide predominantly showed gram-positive colonies.


Bacteria appearing pink or red after the Gram stain procedure.
Researchers noticed a mix of pink gram-negative and purple gram-positive bacteria on the petri dish.


Bacteria that retain violet stain in Gram staining.
When tested, the sample revealed a presence of gram-positive bacteria.


Bacterial classification differentiated by cell wall and staining properties.
The patient's condition was worsened by a gram-negative bacterial infection.


Bacteria lacking an outer lipid membrane.
Gram-positive bacteria tend to be more susceptible to certain antibiotics.


Bacteria with an additional outer lipid membrane.
The antibiotic resistance of some gram-negative bacteria is concerning.


Of, relating to, or being a bacterium that retains the violet stain used in the Gram stain method.


Of, relating to, or being a bacterium that does not retain the violet stain used in the Gram stain method.


That is stained violet by Gram's method


That is not stained violet by Gram's method


(of bacteria) being or relating to a bacterium that retains the violet stain used in Gram's method


(of bacteria) being or relating to a bacterium that does not retain the violet stain used in Gram's method


What do gram-positive and gram-negative terms refer to?

They refer to bacterial classifications based on cell wall structure and Gram staining properties.

Which has a thicker peptidoglycan layer?

Gram-positive bacteria have a thicker peptidoglycan layer.

And how about gram-negative bacteria's appearance post-staining?

Gram-negative bacteria appear pink or red after staining.

How do gram-positive bacteria appear after staining?

Gram-positive bacteria appear purple or violet after staining.

Do both bacteria types cause diseases?

Yes, both gram-positive and gram-negative bacteria can cause various diseases.

Are all pink-staining bacteria gram-negative?

Typically, yes. Pink staining usually indicates gram-negative bacteria.

Can the distinction aid clinical treatments?

Yes, understanding whether bacteria are gram-positive or gram-negative guides antibiotic choices.

Are there exceptions in staining results?

Yes, some bacteria might not fit strictly into the gram-positive or gram-negative classification based on staining.

Are gram-negative bacteria always harmful?

No, not all gram-negative bacteria are pathogenic; some are beneficial.

Which type generally has higher antibiotic resistance?

Gram-negative bacteria often exhibit higher resistance due to their outer lipid membrane.

What color is the original Gram stain?

The original Gram stain is crystal violet or purple.

Do gram-positive bacteria play roles in food production?

Yes, certain gram-positive bacteria are used in producing yogurt, cheese, and other fermented foods.

Can both bacteria types be found in the human body?

Yes, both gram-positive and gram-negative bacteria can be part of the human microbiome.

Is gram-negative bacteria's resistance always due to the outer membrane?

Mostly, but other mechanisms like efflux pumps also contribute to resistance in gram-negative bacteria.

Can the presence of either bacteria type indicate environmental conditions?

Yes, the presence or absence of certain gram-positive or gram-negative bacteria can indicate specific environmental factors.

How was the distinction between gram-positive and gram-negative first made?

It was based on the Gram stain method developed by Hans Christian Gram.

Which bacterial type lacks an outer lipid membrane?

Gram-positive bacteria lack an outer lipid membrane.

Can either bacteria type be used in biotechnological applications?

Yes, both gram-positive and gram-negative bacteria have roles in biotech, from research to product formation.

Is the Gram stain method always accurate?

While generally reliable, some bacterial species might not stain as expected or be decolorized easily.

Why is antibiotic resistance a concern with gram-negative bacteria?

Their outer lipid membrane often acts as a barrier to many antibiotics, leading to resistance.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.

Trending Comparisons

Popular Comparisons

New Comparisons