Gram staining is a method for staining samples of bacteria that differentiates between the two main types of bacterial cell wall.

It is named after the inventor, the Danish scientist Hans Christian Gram (1853-1928), who developed the technique in 1884 to discriminate between pneumococci and Klebsiella pneumoniae bacteria.

Gram Staining a Step by Step Procedure

  1. Heat fix the specimen.
  2. Add a basic dye to stain the sample. Crystal violet or gentian violet are suitable. Allow to stain for 1 minute. The slide should look violet in colour.
  3. Rinse off with water.
  4. Add iodine solution (1% iodine, 2% potassium iodide in water]]) for 1 minute. This acts as a mordant and fixes the dye.
  5. Rinse with water.
  6. Apply 95% ethanol or a mixture of acetone and alcohol several times until no more colour appears to come from the sample.This leaves Gram-positive organisms stained purple and Gram-negative organisms unstained.
  7. Rinse with water.
  8. Apply a suitable counterstain. Opinions vary as to the best choice but suitable stains include safranin or fuchsin.This stains the gram negative organisms.
  9. Blot gently and allow to dry. Do not smear.

Inspect the slide under a
Gram positive organisms will appear blue-black or purple.
Gram negative organisms will appear red.

Gram-positive bacteria have a thick meshlike cell wall made of peptidoglycan which is capable of retaining the violet dye. Gram negative bacteria have a thin cell wall made of a layer of peptidoglycan surrounded by an outer membrane containing lipids.

As a rule of thumb (which has exceptions), Gram-negative bacteria are more dangerous as disease organisms, because their outer membrane acts as "camouflage"; the human body does not contain peptidoglycan and in fact produces an enzyme called lysozyme which attacks the open peptidoglycan layer of Gram-positive bacteria. Gram-positive bacteria are also much more susceptible to penicillin.