Difference Between Gram-Positive and Gram-Negative Bacteria

Bacterias, being one of the smallest microorganisms on the planet, are found in 2 major tapes GRAM-POSITIVE and GRAM-NEGATIVE. Scientists have developed various methods such as genome sequencing, mass spectrometry, polymerase chain reaction (PCR) techniques to differentiate between these teeny-tiny bacterias. Even physical differences such as shape and presence of cell walls are used as parameters of difference. 

Gram-Positive vs Gram-Negative Bacteria

The main difference between gram-positive and gram-negative bacteria sees the presence or absence of cell membrane as a parameter of differentiating. It is the presence of this cell membrane in gram-positive bacteria which makes it different from the gram-negative ones. Lipid is absent in gram-negative bacteria but is present in gram-positive ones.

Gram Positive vs gram Negative

Gram-positive bacteria possess thick cell walls but the cell membrane (also known as the outer membrane) is absent in them. Substances can easily penetrate through gram-positive bacteria because of this absence of cell membrane. These bacteria give rise to purple stains during the gram stain test. They do not contain lipids.

Gram-negative bacteria on the other hand possess a thin cell wall. However, a cell membrane (also known as the outer membrane) is also present in them. Substances cannot penetrate it because of the lipid present in the cell membrane. These bacteria give rise to pink stains during the gram stain test. They contain lipids.

Comparison Table Between Gram-Positive and Gram-Negative Bacteria

Parameters of comparisonGram-positive bacteriaGram-negative bacteria
Gram stain colorIt turns purple after the gram stain testIt turns pink after the gram stain test
Cell membraneThe cell membrane is absent in these. The cell membrane is present in these. 
Cell wall thicknessThe thick, smooth cell wall of 20-80 nanometer thicknessThe thin uneven cell wall of 8-10 nanometer thickness
Penetrating capacityCan be easily penetrated through. It is difficult to be penetrated through. 
Presence of lipidLipid is absentLipid is present in the cell membrane. 

What Is Gram-Positive Bacteria?

Of all bacterias present, even these microorganisms have their types. Gram-positive bacteria are those which have a thick, smooth, and even cell wall of thickness ranging from 20-80 nanometers. The cell membrane (also known as outer membrane) is absent in these. Lipid, which is mostly enclosed by the cell membrane, is also absent. Structures such as flagella or fimbriae (also known as pili) are present to help in locomotion. It generally consists of a single-layered outer part, with only the cell wall.

These bacteria can be easily penetrated by foreign substances due to the absence of cell membrane and are thus easily destroyed. During the Gram Stain Test, where a sample would be placed under a slide and treated with gram stain, it turns purple, which is observed through a microscope. This change into purple happens, because of the absence of the cell membrane, which causes the dye to easily penetrate its inner part.

It mostly contains exotoxins which are unstable and intolerant to heat, thus they diffuse into the surrounding medium. These bacteria might cause minor gum diseases, diphtheria, acne, leprosy, and various bacterial skin infections and inflammations. Some gram-positive bacteria help in nitrogen fixation namely Actinobacteria.

Various examples of these types of bacteria include – Staphylococcus Aureus, Streptococcus Pneumoniac, Enterococcus Faecalis, Hay Bacillus, Bacillus Cereus, Anthrax bacterium, Mycoplasma Hominis, Milky spore, Ureaplasma Parvum, and Mycobacterium large.

What Is Gram-Negative Bacteria?

Gram-negative bacteria have many of their characteristics opposite to those of gram-positive bacteria. It has a thin, uneven, multilayered cell wall, of thickness ranging from 8-10 nanometers. The cell membrane (also known as outer membrane) is present in these. Lipid is enclosed within the cell membrane. Locomotory organs such as flagella or fimbriae (also known as pili) are present. The outer layer is usually triple-layered with the cell wall, cell membrane, and lipid layer.

These bacteria cannot be easily penetrated through by foreign substances, because of the presence of lipid-filled cell membrane. These are thus harder to be destroyed. During the gram stain test, when the sample is treated with the stain, it turns pink when observed under a microscope. The dye doesn’t penetrate through the cell membrane so the inner part doesn’t get stained. 

It contains both Exotoxins, which are proteins that aren’t heat resistant. And Endotoxins, which are heat-resistant lipopolysaccharide (LPs) proteins that only diffuse after the death of bacteria. It contains many types of amino acids.

Examples of gram-negative bacteria include E. Colli, Pseudomonas aeruginosa, Meningococcus, Proteus Vulgaris, Chlamydia Pneumoniae. Gram-negative bacterias can cause some harmful ailments such as low blood pressure, low oxygen levels, Cholera, and various infections. 

Main Differences Between Gram-Positive and Gram-Negative Bacteria

  1.  During the gram stain test, gram-positive bacteria change their color to purple whereas gram-negative bacteria turn pink. 
  2. The cell membrane (also known as the outer membrane) is absent in gram-positive bacteria. But in the case of gram-negative bacteria, a cell membrane is present. 
  3. The cell wall is thick, with thickness up to 20-80 nanometers, and smooth in gram-positive bacteria. Whereas it is thin of the thickness of 8-10 nanometers and uneven in gram-negative bacteria. 
  4. Gram-positive bacteria have a high penetrating capacity as the cell membrane is absent and foreign substances penetrate it easily. However, gram-negative bacteria have a cell membrane which makes it difficult to be penetrated. 
  5. Lipid layer on absent gram-positive bacteria. Whereas in gram-negative bacteria, the lipid layer is present, this lipid layer might undergo toxic reactions and cause diseases. 


After discussing all the points of difference in detail, we can easily find out the difference between these two different types of bacteria by the gram stain test. The test determines the presence or absence of cell membrane and thus identifies the type of bacteria we are dealing with, cell membrane if absent means gram-positive. Cell membrane if present means that it is gram-negative bacteria. Also the presence of toxins, cell wall thickness are other determining factors. 

Several diseases caused by these bacteria can be easily identified. Not all bacteria are harmful, some bacteria also help to maintain balance in our body and to maintain good health.  Disease-causing bacteria that are hard to destroy are now being dealt with scientific weapons and thus mankind is saved. 


  1. https://link.springer.com/article/10.1007/BF00498806
  2. https://link.springer.com/content/pdf/10.1007/BF01026102.pdf..pdf
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