Gram Negative Vs. Gram Positive Bacteria

Bacteria are microscopic organisms, typically a few micrometers in length. They have a wide range of shapes, ranging from spheres to rods and spirals. They are present in most habitats on Earth and grow in soil, water, organic matter and also in plants and animals. They are broadly classified as Gram-negative and Gram-positive depending on their cell wall structure and a few other characteristics. The gram staining method is widely used to differentiate between these two bacterial species. It is an important step in the process that is routinely followed to identify a bacterial species as gram positive or gram negative.

When carrying out this study, the property of the species to take on a particular color or stain is used as the basis of differentiation. This method is named after Hans Christian Gram, its inventor. This laboratory technique is mainly used to test bodily fluids under conditions where an infection is suspected and the time factor is crucial. It works faster for the purpose of differentiating in many conditions.

Characteristics
Apart from the effect of gram staining, gram-negative and gram-positive bacteria differ in various other characteristics. Let us take a look at them.

Wall Structure: The main difference between gram-negative and gram-positive bacteria has to do with the arrangement of the outer structure of the cell.

Effect of Dye: A crystal violet dye solution is added to the bacteria to find their type. This process is known as gram staining.

Effect of Antibiotics: Identification of bacterial type is imperative for the treatment of medical conditions, since different bacteria have differing responses to antibiotics like penicillin.

Flagellum: A flagellum is a hair-like structure that is used by the bacteria to move from one place to another.

Teichoic Acids: Teichoic acid is a strong acidic polymer found in the wall of a cell. Its main function is to provide rigidity to the cell wall.

Liproproteins: Bacterial lipoproteins are a set of membrane proteins which play an important role in various bacterial diseases caused by gram-negative and gram-positive bacteria.

Periplasmic Space: There is a space between the layers of peptidoglycan and the secondary cell membrane which is known as the periplasmic space.

Comparison
The following table gives a detailed comparison between the gram-negative and gram-positive bacteria.

Characteristic	Gram-negative Bacteria	Gram-positive Bacteria
Wall Structure	Gram-negative bacteria have a lipopolysaccharide exterior cell wall which is thin.	The peptidoglycan layer or the mesh-like structure that is seen outside the plasma membrane in gram positive bacteria is much thicker. It is made up of close to twenty times the amount of peptidoglycan that is present in gram negative bacteria.
Effect of Dye	Gram-negative bacteria do not retain the crystal violet dye and react only with a counterstain. These generally stain pink.	Gram-positive bacteria retain the crystal violet dye and change into purple during staining identification.
Effect of Antibiotics	Bacterial cells that are gram negative are resistant to penicillin. These cells contain an endotoxin - LPS, which forms a coating on the outer cell wall surface and protects the inner cell structures from invasion.	Gram-positive bacteria can be neutralized by the administration of penicillin and is still used for treating many gram-positive bacterial infections.
Flagellum	If present, the flagellum in gram-negative bacteria has four supporting rings namely L ring, P ring, M ring and S ring.	The flagellum in gram-positive bacteria has two supporting rings, one in the peptidoglycan layer and the other in the plasma membrane. Since these bacteria already have a thick cell wall which is enough for support, the two additional rings are not required.
Teichoic Acids	No teichoic acids present.	Teichoic acids are present.
Liproproteins	Lipoproteins are attached to the polysaccharide backbone.	No lipoproteins present.
Periplasmic Space	Periplasmic space is present.	It is absent in gram-positive bacteria.

Gram staining method still continues to be the first step for many routine procedures to identify bacterial species, but there are some organisms that are gram variable, or gram indeterminate. They can therefore not be subjected to this test. In addition, this test has now mainly been replaced by more sophisticated methods which use genetic sequences and molecular structure, widely recognized as more reliable methods of species identification.