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The Gram’s staining (Differential staining) method is responsible for classifying the bacteria into two broad groups i.e., Gram Positive bacteria and Gram Negative bacteria according to the stains they take up during the staining procedure.

The bacteria which take up the count stain (Basic Fuchsin, Saffron) and are stained pink or red in colour are called as Gram-negative bacteria. The gram negative bacteria cell wall is thin, complex, multilayered and contains relatively large lipid content in addition to proteins and mucopeptides. The high amount of lipids are readily dissolved by alcohol, resulting in the formation of pores in the cell wall which causes the Crystal-violet iodine complex (CVI) to leak out and results in decolourisation of the cell which later takes up the counter stain and appears red or pink in colour.

Some examples of Gram Negative bacteria are: Escherichia coli, Salmonella typhi, Haemophilus influenza, Proteus vulgaris, Neisseria gonorrhoeae

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