Difference Between Gram positive and Gram negative Bacteria

What are Gram positive and Gram negative bacteria?

In a typical bacterial cell, there are three layers :

  • The innermost layer is the plasma membrane which directs what enters and exits the cell; phospholipid bilayer with some protein scattered between it

  • The middle layer is the Cell wall that provides structural support to the cell and is made up of peptidoglycan or polysaccharide. The cell wall of bacteria Gram-positive and negative bacteria differ in their thickness. It is an important layer to understand the structure and difference between Gram-positive and negative bacteria, which we will understand later in this write-up.

  • The third layer is the Capsule which is the sticky outer layer for attachment and protection.

The major differences between Gram positive and Gram negative bacteria can be understood with the structural differences and Gram staining differences between them.

Highlighting the Difference Between Gram Positive and Gram Negative Bacteria


Gram Positive Bacteria

Gram Negative Bacteria


It possesses an inner plasma membrane.

It also possesses an inner plasma membrane.

Cell Wall

It has a thickly layered peptidoglycan cell wall.

It has a thin layered peptidoglycan cell wall.

Second phospholipid plasma membrane

It is absent in Gram positive bacteria.

It has an additional outer phospholipid plasma membrane.


Outer capsule present

Outer capsule present

Treatment possibility

It is more easily treatable with Antibiotics due to the absence of a second phospholipid bilayer. 

It is harder to treat with antibiotics as it is difficult for antibiotics to enter the Gram negative cell.

Gram staining results

It appears purple/violet under a microscope after performing a Gram stain experiment.

It appears red/pink underneath the microscope after performing a Gram stain experiment.

Examples of Gram positive bacteria and Gram negative bacteria

Staphylococcus and Streptococcus.

Escherichia and Salmonella.

Toxins Produced



Lipid content

Very low

It has 20-30% of lipid content in the cell.

Flagella structure

It consists of two rings in the basal body.

It has four rings in the basal body.

Teichoic acids




More prominent

Less prominent


Cocci or spore-forming rods are present.

Non-spore forming rods present.

Gram Stain Process

It is a purpose that helps us to identify the type of infectious bacteria for proper diagnosis and treatment based upon outcome. It helps us to highlight the Gram positive and gram negative bacteria difference.

Steps of Gram Staining

  1. Take a glass slide and smear bacteria samples from the culture on it with the help of an inoculation loop. 

  2. Fix the bacteria to the slide to avoid it getting washed away in the later steps, this is done by heat fixing the glass slide over a flame.

  3. Add a few drops of crystal violet to the bacteria sample.

  4. Add iodine molecules to the sample; it is golden brownish in color.

  5. Here, the crystal violet and iodine molecules bind together.

  6. Now, wash the bacteria with alcohol which is a de-staining chemical. 

A major happening in the Gram positive bacteria takes place which is when the capsule of the cell gets washed away and shrinks the cell. And in Gram negative bacteria, the second plasma membrane layer gets washed away along with the capsule due to the dehydrative nature of the alcohol. As the Gram -ve bacteria have a thin cell wall left, the crystal violet-iodine molecules also get washed away, and hence the cell loses its color. On the other hand, Gram +ve bacteria has retained the colour from crystal violet dye and therefore appears purple violet colour under the microscope.

  1. The next step is to apply safranin to the bacterial samples; safranin is a biological stain, pinkish in colour.

  2. Results: The results for Gram positive vs. Gram negative bacteria will be different as mentioned below.

What is Gram positive bacteria result after staining?

Now under observation underneath the microscope, Gram positive bacteria appear purple in appearance as the pinkish colour of safranin is overshadowed by the deep colour of crystal violet dye. 

What is Gram negative bacteria result after staining?

Gram negative bacteria will appear pink in appearance because of safranin only left for imparting the colour.

FAQ (Frequently Asked Questions)

1. Differentiate between gram positive and negative bacteria.

There are structural differences between Gram positive and Gram negative bacteria that makes them appear different after Gram staining experiment. The cell wall of the former kind is thicker peptidoglycan wall whereas that of latter is thinner. The former bacterial cell has a capsule after the cell wall whereas a second plasma membrane layer is present in the latter kind of bacterial cell. This makes it harder for antibiotics to get inside the Gram negative bacteria cell and kill it. So, it is difficult to treat if infected with Gram negative bacteria like Salmonella, Pseudomonas, Shigella, and other Enterobacteriaceae, Moraxella, Helicobacter, Stenotrophomonas, acetic acid bacteria, Legionella, etc. On the other hand, Gram positive bacteria like cocci, bacilli, Staphylococcus aureus, and  Streptococcus pneumonia can be treated easily after their diagnosis.

2. What are the infections which are caused by gram-positive bacteria?

Some of the infections caused by Gram positive bacteria include Urinary Tract Infections and pneumococcal infections. These are caused commonly in people who are elderly or pregnant.

3. What are the infections which are caused by gram-negative bacteria?

Some of the infections caused by Gram negative bacteria include bloodstream infections, wound infections, and meningitis.

4. Which are easily treated, Gram positive or Gram negative bacterial infections?

Gram positive bacterial infections can be treated with antibiotics easily as they possess cell walls that can absorb cleaning medications. They also do not have a second phospholipid plasma membrane before the capsule in their cellular structure as compared to the Gram negative bacteria where it is present. This makes it easier for antibiotics to work on the former type.

5. What is the major difference between Gram positive and Gram negative bacteria?

The major difference can be found out after Gram staining with crystal violet and iodine and safranin at the end where Gram positive bacteria are purple in colour under the microscope and the latter are pinkish in appearance.