Lecture notes in Microbiology

Difference Between Gram Positive and Gram Negative Bacteria


Gram positive and gram negative staining

Gram Positive Bacteria Vs Gram Negative Bacteria
(Similarities and Differences Between Gram Positive and Gram Negative Bacteria)

What is Grams staining?

Christian Gram, a Danish Physician in 1884 developed a staining technique to distinguish two types of bacteria. The two categories of bacteria based on gram staining are Gram positive bacteria and Gram negative bacteria. Bacteria are first stained with crystal violet or gentian violet. All bacterial cells will stain blue or purple colour with crystal violet solution. Then the bacterial cells are treated with iodine solution (Lugol’s iodine) solution and washed with alcohol (de-staining solution).  Those bacteria which retain the blue or purple colour of crystal violet are called Gram positive bacteria and those bacteria which loose the colour of crystal violet after washing with de-staining solution is called Gram Negative bacteria. Gram negative bacteria are later stained with safranin or fuchsin for observation under microscope. Gram negative bacteria after safranin or fuchsin staining will appear red or pink colour. Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting the properties of peptidoglycan. Gram staining method is useful in differentiating majority of bacterial species into two broad categories. Even though all bacterial species cannot be differentiated based on gram staining technique, this method has immense application in clinical diagnostics and biological researches.


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Similarities between Gram Positive and Gram Negative Bacteria

Ø  Both are bacterial cells

Ø  Both groups are prokaryotic

Ø  Both lack membrane bounded organelles

Ø  Both groups have covalently closed circular DNA as the genetic material

Ø  Both groups contain extra-chromosomal genetic materials (plasmids)

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