Acridine Orange Staining: Principle, Procedure, Results and Applications

Acridine orange is a dye that intercalates or binds with the nucleic acid ( either DNA or RNA) present  in organisms and fluoresce to emit various colors that help in differentiation of cellular organells. This binding is the result  the electrostatic interactions of acridine molecule between the nucleic acid base pairs. Acridine Orange (AO), due to its metachromatic properties, is commonly used in fluorescence microscopy and flow cytometry analysis of cellular physiology and cell cycle status, including the fluorescent microscopic examination of microorganisms.

Principle : Acridine orange is a cell-permeable,nucleic acid selective dye that emits green fluorescence when bound to dsDNA(at 520 ) and red fluorescence when bound to ssDNA or RNA(at 650 nm).Since it is a cationic dye,it also  enter acidic compartments such as lysosomes  which in  low pH conditions, will emit orange light.

Staining procedure:

Staining procedures vary according to its use

  1. For staining clinical specimen with acridine orange at low pH (Acridine orange Acid Stain)
  1. Prepare a smear in a clean grease free slide and allow it to air dry.
  2. The slide is then  fixed with methanol and  dried again.
  3. It is then put in  trough with acridine orange staining working solution (i.e 0.01 per cent).
  4. After 2 minutes of staining, the slides are washed gently with water and dried and then examined in a fluorescent microscope.

    Observance: Bacteria stain orange against a green to yellow background of human cells and debris.

  1. For staining cells for analysis by flow cytometry.
    Requirements: 0.1M Citric Acid (dissolve 1.921g per 100ml distilled water) ,0.2M Dibasic Sodium Phosphate  (dissolve 2.839g per 100ml distilled water) ,Triton X-100 (Baker),0.5M EDTA, Sodium chloride(NaCl), Acridine Orange (Powder) and Sucrose.
  2. Preparation of reagents:
    • Stock Buffer I :20mM Citrate-Phosphate, pH 3.0, 0.1mM EDTA, 0.2M Sucrose, 0.1% Triton X-100
      (To 125ml distilled water add 40µl 0.5M EDTA, 26.48ml 0.1M Citric Acid, 6.85ml 0.2M Dibasic Sodium Phosphate, 13.69g Sucrose, 0.2ml Triton X-100 .QS to 200ml and 0.2µ filter. Store at 4oC)
      Stock Buffer II :10mM Citrate-Phosphate, pH 3.8, 0.1M NaCl
    • (To 150 ml distilled water add 9.92ml 0.1M Citric Acid, 5.46ml 0.2M Dibasic Sodium Phosphate, 1.7g NaCl. QS to 200ml and 0.2m filter. Store at 4oC)

Staining Procedure :

  1. Make a 2mg/ml solution of Acridine orange in distilled water and dilute to 1:100 in Buffer II
  2. Aliquot cells: 105- 106 in 100µl PBS or media .
  3. Add Buffer I (0.5ml) at room temp, agitate to suspend .
  4. Add Buffer II + AO (0.5ml) at room temp, agitate to suspend.
  5. Run on flow cytometer. Excitation 488 nm; dot plot of green fluorescence at 530nm versus red fluorescence >600 nm).


Green fluorescence when bound to dsDNA and red fluorescence when bound to ssDNA or RNA.


 Researches showed that

Acridine orange staining is a sensitive, rapid and reliable method for detecting bacteria in blood cultures early during incubation and can be substituted for blind subcultures.

Acridine orange is better than Gram stain in cases with low amounts of organisms.