Gram Staining

Gram stain

The Gram staining method is named after the Danish bacteriologist Hans Christian Gram (1853 –1938) who originally devised it in 1882 (but published in 1884), to discriminate between pneumococci and Klebsiella pneumoniae bacteria in lung tissue. It is a differential staining method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative) based on the chemical and physical properties of their cell walls. This reaction divides the eubacteria into two fundamental groups according to their stainability and is one of the basic foundations on which bacterial identification is built. Gram staining is not used to classify archaea, since these microorganisms give very variable responses.

Gram staining consists of four components:
Gram staining generally consists of four components, that are;
Primary stain (Crystal violet, methyl violet or Gentian violet)
Mordant (Gram's Iodine)
Decolourizer (ethyl alcohol, acetone or 1:1 ethanol-acetone mixture)
Counterstain (Dilute carbol fuchsin, safranin or neutral red)
                                                                                Staining Results of bacteria
The original description of staining technique by Christian Gram in a publication titled "The differential staining of Schizomycetes in tissue sections and in dried preparations" in Fortschitte der Medicin; 1884, Vol. 2, pages 185-189 was slightly different from what we use today. The primary stain used was aniline gentian violet, mordant was Lugol's iodine (iodine-potassium iodide in water), decolorizer was absolute alcohol and bismark brown was the counterstain.

Following are the steps of Gram Staining;
  • The smear on a glass slide is covered with few drops of one of the primary stains. Gentian violet is a mixture of methyl violet and crystal violet. The primary stain renders all the bacteria uniformly violet. After a minute of exposure to the staining solution, the slide is washed in water. 
  • The smear is treated with few drop of Gram's Iodine and allowed to act for a minute. This results in formation of a dye-iodine complex in the cytoplasm. Gram's iodine serves as a mordant. 
  • The slide is again washed in water and then decolorized in absolute ethyl alcohol or acetone. A mixture of ecetone-ethyl alcohol (1:1) can also be used for decolorization. The process of decolorization is fairly quick and should not exceed 30 seconds for thin smears. Acetone is a potent decolorizer and when used alone can decolorize the smear in 2-3 seconds. A mixture of ethanol and acetone acts more slowly than pure acetone. Decolorization is the most crucial part of Gram staining and errors can occur here. Prolonged decolorization can lead to over-decolorized smear and a very short decolorization period may lead to under-decolorized smear.
  • After the smear is decolorized, it is washed in water without any delay. The smear is finally treated with few drops of counterstain such as dilute carbol fuchsin, neutral red or safranin. 
  • The slide is washed in water; excess water is removed using a blotting paper, dried in air and heat fixed before observing under microscope.
  • Those bacteria that hold on to primary dye-iodine complex and remain violet are called Gram positive and those which get decolorized and subsequently take up counterstain (pink/red) are called Gram negative. 
  • Basic fuchsin (present in dilute carbol fuchsin) stains many Gram negative bacteria more intensely than does safranin, making them easier to see. Some bacteria which are poorly stained by safranin, such as Haemophilus spp., Legionella spp., and some anaerobic bacteria, are readily stained by basic fuchsin.
  • In order to ascertain if the staining procedure was satisfactorily conducted, a control smear of known Gram positive organism (e.g., Staphylococcus aureus) and a known gram negative organism (Escherichia coli) must be stained simultaneously. While the fibrin in a clinical specimen may appear gram positive, the pus cells and epithelial cells are always gram negative.

Mechanism of Gram reaction:
Mechanism of Gram Staining includes Various theories that have been proposed to explain why some bacteria retain the dye and some don't. Theories such as differences in cytoplasmic pH (2 in case of Gram positive bacteria and 3 in case of Gram negative bacteria), and presence of Magnesium ribonucleate in Gram positive bacteria and its absence in Gram negative bacteria have not received widespread acceptance. The thickness of Gram positive cell wall and presence of more lipids in Gram negative cell walls have been more acceptable reasons for Gram stain reactions.

It is believed that the positively charged crystal violet pass through the cell wall and cell membrane and binds to negatively charged components inside the cell. Addition of negatively charged iodine (in the mordant) binds to the positively charged dye and forms a large dye-iodine complex within the cell. Crystal violet (hexamethyl-para-rosaniline chloride) interacts with aqueous KI-I2 via a simple anion exchange to produce a chemical precipitate. The small chloride anion is replaced by the bulkier iodide, and the complex thus formed becomes insoluble in water. During decolorization, alcohol dissolves the lipid present in the outer membrane of Gram negative bacteria and it leaches the dye-iodine complex out of the cell. A thin layer of peptidoglycan does not offer much resistance either. The dye-iodine complexes are washed from the Gram negative cell along with the outer membrane. Hence Gram negative cells readily get decolorized. On the other hand Gram positive cells become dehydrated from the ethanol treatment, closing the pores as the cell wall shrinks during dehydration. The dye-iodine complex gets trapped inside the thick peptidoglycan layer and does not get decolorized

Limitations of Gram staining:
The limitation of Gram Staining is some Gram-positive bacteria may lose the stain easily and therefore appear as a mixture of Gram-positive and Gram-negative bacteria (Gram-variable). When over-decolorized, even Gram positive bacteria may appear pink and when under-decolorized gram negative bacteria may appear Gram positive.

The Gram reaction also depends on the age of the cell. Old cultures of Gram positive bacteria (where cell walls may be weakened) may readily get decolorized. Gram positive cells affected by cell wall active agents such as lysozyme or antibiotics may become Gram negative. Gram-positive bacteria such Actinomyces, Arthobacter, Corynebacterium, Mycobacterium, and Propionibacterium have cell walls particularly sensitive to breakage during cell division, resulting in Gram-negative staining of these cells. In cultures of Bacillus, and Clostridium a decrease in peptidoglycan thickness during cell growth may cause some of them to appear Gram negative.

Certain group of bacteria can display variable response to the stain, which can be due to growth stress (e.g., unsuitable nutrients, temperatures, pHs, or electrolytes) that results in a number of nonviable, gram-negative cells in a gram positive culture, but certain bacterial species are known for their gram variability even under optimal growth conditions. Some bacteria tend to appear Gram negative when grown in acidic medium.

Loss of cell walls in Gram positive bacteria may render them Gram negative (L-forms). Bacteria totally devoid of cell wall (Mycoplasma) are always Gram negative. Bacteria such as Mycobacterium that have extra waxy content in their cell wall are difficult to stain. Small and slender bacteria such as Treponema, Chlamydia, Rickettsia are often difficult to stain by Gram's method. Gram positive bacteria that have been phagocytosed by polymorphs may also appear Gram negative.

Modifications of Gram stain:

There have been several modifications of Gram's stain. These are:

1. Kopeloff and Beerman's modification: Primary stain solution consists of freshly constituted methyl violet with sodium bicarbonate in distilled water. Mordant consists of iodine dissolved in 4% NaOH solution. Decolorization is either using acetone alone or a mixture of acetone and ethanol. Basic fuchsin is used to counterstain the smear. This method may be modified to stain tissue sections.

2. Jensen's modification: This method involves use to methyl violet as primary stain, iodine and potassium iodide in water as mordant, absolute alcohol as decolorizer and neutral red as counterstain. For Neisseria spp, Sandiford's counterstain is useful.

3. Weigert's modification: This modification is particularly useful for staining tissue sections. The primary stain carbol gentian violet is prepared using saturate alcoholic solution of gentian violet and 5% phenol solution. Gram's iodine is used as a mordant and aniline-xylol is used as a decolorizer. The counterstain carmalum (carminic acid and potassium alum in water), however is used ahead of primary stain. This method may be used to stain Pneumocystis cysts.

4) Preston and Morrell's modification: The primary stain used in this modification is ammonium oxalate-crystal violet. The smear is washed in Lugol's iodine and further treated with iodine solution. The smear is decolorized using iodine-acetone decolorizer and counterstained using dilute carbol fuchsin solution. This method has been further modified to overcome the irritating iodine in aerosols by reducing the iodine concentration to one-tenth and shortening the duration of decolorization to ten seconds. 

Applications of Gram staining:

Differentiation of bacteria into Gram positive and Gram negative is the first step towards classification of bacteria.
It also the first step towards identification of bacteria in cultures.
Observation of bacteria in clinical specimens provides a vital clue in the diagnosis of infectious diseases.
Useful in estimation of total count of bacteria.
Empirical choice of antibiotics can be made on the basis of Gram stain’s report.
Choice of culture media for inoculation can be made empirically based on Gram’s stain report.


Although Gram stain is useful in staining bacteria, certain fungi such as Candida and Cryptococcus are observed as Gram positive yeasts.
Half-Gram stain refers to modified staining technique, where the smear is neither decolorized nor counterstained. It is useful to stain a known Gram positive bacterium.
Rapid Gram stain refers to quickened technique where the smear is exposed to only 30 seconds instead of one minute.
In specimen such as sputum, capsulated bacteria may stand out as clear spaces between the bacterium and the pink (mucus) background.
The spores may stand out as clear, unstained region in sporing bacteria while doing Gram Staining.


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