Difference Between Gram-positive and Gram-negative


Main Difference

Bacteria are of different types depending on their location and the families they belong to. There are several ways to differentiate them as well, and one of such method is called Gram staining which is a four step method used to enhance their microscopic image. By this, Christian Gram introduced a method of dividing bacteria into two different types. The stain which is used is mostly an alkaline solution and the basis of it along with the cell structure bacteria are grouped into two different types. These are obviously distinct from each other and have various characteristics which are there for categorizing them. Some of those differences are given here to help to erase the confusion between them. The best way to know the difference is one the basis of this test, the bacteria which is able to keep its original color after the grain test is called gram-positive bacteria while the one which loses their color during the test are known as Gram-negative bacteria. There are many other differences as well, for example, the cell wall of gram-positive bacteria is thicker and usually 20-25 nm while the cell wall of gram-negative bacteria is less thick and around 8-11 nm thick. Another difference on the wall is that for the first one has a smooth surface while the latter one has a curvy surface. Other factors that help in separating the two include that gram-positive bacteria there is no periplasmic space present while the outer membrane along with porins is absent. In the case of gram-negative bacteria, the periplasmic space, and the outer layer is present which also contains the porins. Even during the reaction telling the difference between them is possible as gram positive bacteria turns blue or purple in color while the gram-negative bacteria turns reddish brown in color. Usually, the resistance in gram-positive bacteria is higher since it is able to stop the drying and different reactions while resistance is lower in gram-negative bacteria. Mesosomes are more prominent in the first type of bacteria because of which the flagellar structure consists of dual rings while mesosomes are less noticeable in the latter one, and therefore it has a four-ring structure. Some of the other differences between them will be mentioned in the last section.

Comparison Chart

DefinitionBacteria which are able to maintain their original color during the stain testThe ones which are not able to preserve their color during this process
ColorTurns into a blue or purple color substanceTurns into either red or brownish red colored substance
MembraneNo outer membraneDistinct outer layer
Sizecell wall is thick and around 20-25 nmcell wall is around 8-11 nm

Definition of Gram-positive Bacteria

During the gram test if the bacteria are able to maintain their distinct color they are termed as gram positive bacteria, but that is not the only way on which they can be categorized. There are many other properties by which they are given their name. For example, they do not have an outer membrane and during the gram reaction, they are able to maintain a blueish purple color which gives them the distinction. They do have a cell wall present which is ticker up to a few Nanometers and is smoother as compared to other parts. They also have a flagellar structure in which there are two rings which form a body. It also produces toxins in the shape of exotoxins while they also have higher resistance to drying. In the case of chemical reactions, they also show a good response and resist any change. Many other characteristics such as the lack of porins and the surface of the membrane are also its distinct features.


Definition of Gram-negative Bacteria

During the gram test if the bacteria are not able to maintain their distinct color they are termed as gram-negative bacteria, but that is not the only way on which they are differentiated. Some of the other properties such as the color they achieve during the staining process give them the features. They obtain a red color during the process and have a flagellar structure with four rings. They do have an outer membrane which contains a considerable amount of porins, but teichoic acids are absent along with a little presence of murein which is just 20%. Their cell wall is also thin as compared to the other type of bacteria and can range between 7-11 nm. They do have a higher amount of lipid content on the wall which is not smooth and gives a rougher wavier look. They also are not good at resisting drying and showing a reaction to different chemical activities while the mesosomes are clearly visible in the gram-negative bacteria.

Differences in a Nutshell

  1. Bacteria which are able to maintain their original color during the stain test are known as gram-positive bacteria while the ones which are not able to preserve their color during this process are called Gram-negative
  2. During the stain reaction, the gram-positive bacteria turns into a blue or purple color substance while the gram-negative bacteria turns into either red or brownish red colored substance.
  3. The majority of pathogenic bacteria are known as Gram-negative while few of the ones are called gram positive.
  4. There are teichoic acids in the wall of gram-positive bacteria while it is absent in gram-negative
  5. There are mesosomes in the gram-positive bacteria while they are less prominent in gram-negative
  6. There is no outer membrane in the gram-positive bacteria while there is a distinct outer layer of the gram-negative
  7. The cell wall of gram positive bacteria is thick and around 20-25 nm while the cell wall for gram-negative bacteria is around 8-11 nm.
  8. The cell wall is smoother and contains up to 80% murein for gram-positive bacteria while the cell wall is wavy and contains up to 20% murein in the case of gram-negative.

Comparison Video

YouTube video


All in all, this article will help in developing a clear understanding of how things work when it comes to Gram-positive and Gram-negative. Weights can be confusing at times since there are many units and that can create issues for people who do not have a proper understanding of the topics and related activities.

Harlon Moss

Harlon currently works as a quality moderator and content writer for Difference Wiki. He graduated from the University of California in 2010 with a degree in Computer Science. Follow him on Twitter @HarlonMoss