Gram staining  is a special technique which is used to stain bacteria. This technique was developed by Christian Gram in 1884. The stain  stain used in Gram staining is called Gram stain. Chemically Gram stain is a weakly alkaline solution of crystal violet or gentian violet.
On the basis of cell wall structure and its stain ability wit Gram stain, bacteria are grouped into two categories. They are Gram positive bacteria and Gram negative bacteria. 
The bacteria which retain the colour of the stain are called Gram positive bacteria. Examples of Gram positive bacteria

The bacteria which lose the colour of the stain are called Gram negative bacteria. Examples of Gram negative bacteria
Gram Positive and Gram Negative Bacteria
Gram Positive vs Gram negative Bacteria
Gram Positive Bacteria Gram Negative Bacteria
The bacteria  remain coloured with Gram staining even after washing with alcohol or acetone. The bacteria do not retain the stain when washed with alcohol or acetone.
Outer membrane is absent. Outer membrane is present.
Cell wall is 20-30 nm thick. Cell wall is 8-12 nm thick.
The wall is smooth The wall is wavy and comes in contact with plasma membrane only at a few loci.
The wall contains 70-80% murein. The wall contains 10-20% murein.
The lipid content in the wall is very low. The lipid content in the wall is 20-30%.
Porin are absent. Porins or hydrophilic channels occur in outer membrane.
Cell wall contains teichoic acids.Gram positive Cell wall Teichoic acids are absent. Gram negative cell wall
Basal body of the flagellum contains  two rings. Basal body of the flagellum has four rings.
Mesosomes are quite prominent. Mesosomes are less prominent.
A few pathogenic bacteria belong to Gram positive group. Most of the pathogenic bacteria belong to Gram negative group.
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