# Negative Staining – Principle, Procedure, Results and Uses

> Negative staining in microbiology, including its principle, dyes, laboratory procedure, microscopic results, applications, advantages, and limitations.

Canonical URL: https://biologynotesonline.com/negative-staining/
Author: Sourav Pan
Last updated: November 13, 2026

![Negative Staining – Principle, Procedure, Results and Uses](https://biologynotesonline.com/wp-content/uploads/2025/12/principle-of-negative-staining.webp)

Negative staining is used in microbiology laboratories for the examination of bacterial smears under a bright-field microscope. Unstained bacterial cells are nearly colourless. They have very little contrast with the surrounding medium and are difficult to observe clearly.

In routine bacterial staining, the smear is often heat-fixed before the stain is applied. Excessive heating may cause the bacterial cells to shrink or become distorted, affecting their natural shape and size.

## What is Negative Staining?

Negative staining or indirect staining is a staining method in which the surrounding background is stained while the cells remain unstained or lightly coloured. The bacterial cells appear clear or pale against a dark background when observed under a light microscope.

Negative staining is used for the microscopic examination of microorganisms, particularly bacteria, to observe their shape and arrangement.

It is also considered a form of simple staining when only one dye is used. Unlike direct positive staining, where the bacterial cells take up the dye, negative staining colours the surrounding area.

The term negative staining is also used in electron microscopy. In this method, electron-dense stains are used to surround the specimen and produce contrast under the electron microscope.

## Objectives of Negative Staining

The following are the objectives of negative staining:

- To prepare bacterial smears using the negative staining method.

- To observe bacterial cells against a stained background.

- To study the shape and arrangement of bacterial cells.

- To determine the size of bacterial cells without distortion caused by heat fixation.

- To examine delicate bacterial cells without heat fixation.

- To detect the presence of capsules around certain bacteria.

- To distinguish negative staining from direct positive staining.

## Principle of Negative Staining

![Negatively charged nigrosin dye species remain around a bacterial cell, producing pale bacterial rods against a dark stained background.](https://biologynotesonline.com/wp-content/uploads/2025/12/principle-of-negative-staining-1024x576.webp)Negatively charged nigrosin dye species remain around a bacterial cell, producing pale bacterial rods against a dark stained background.

Most bacterial cells have a negatively charged surface (under the staining conditions we're talking about). [Acidic dyes](https://biologynotesonline.com/types-of-stains-used-in-microbiology/) such as nigrosin have negatively charged chromophores. When you add the dye to the smear the like charges repel and the dye molecules don't readily bind to the cells.

The dye stays in the surrounding area and stains the background rather than the bacterial cells. The bacterial cells themselves appear clear against the dark background when viewed under the light microscope.

India ink consists of fine carbon particles in a liquid. The particles remain around the bacterial cells and give a dark background. It's different from nigrosin in that it doesn't rely on the repulsion between the anionic dye molecules in solution and the bacterial cell surface.

In negative staining heat fixation is normally not done. The smear is simply allowed to air dry. Heat can cause the cells to shrink and distort, altering their natural size and shape.

## Dyes Used in Negative Staining

Nigrosin and India ink are the two stains most commonly used for negative staining of bacteria. Other acidic dyes (e.g. Congo red, eosin) are also used in various lab techniques.

Nigrosin

Nigrosin is an acidic dye which contains negatively charged chromophores and doesn't readily bind to bacterial cells as their surfaces are also generally negatively charged. When you mix nigrosin with a bacterial smear it will stain the area around the cells dark grey or black and the cells themselves will remain clear against the stained background.

Nigrosin is a fairly common stain in the microbiology lab used to look at the morphology (shape, size and arrangement) of bacteria. It's available in a number of forms, most commonly as an aqueous solution. The concentration used will be protocol dependent and different commercial stains will of course have different formulations.

India Ink

India ink is a colloidal preparation consisting of very fine pigment particles (primarily carbon) suspended in a liquid. The pigment particles are not dissolved in the liquid as they are in water-soluble acidic dyes.

The particles remain associated with the bacterial cells and provide a dark background. They are also excluded from the capsule of some organisms, leaving a clear space around the cell.

India ink is used for the demonstration of bacterial capsules and is also used to show the capsule of encapsulated yeasts (e.g. Cryptococcus neoformans). The capsule appears as a clear halo between the cell and the dark background.

Other Negative Stains

Congo red is another acidic dye used for negative staining in some of the bacterial and capsule staining methods. It normally gives a red background but the colour can change to blue under acidic conditions. In Maneval's capsule stain for instance the acidic staining solution will turn the background blue.

Eosin is also an acidic dye and can be used for negative staining. It's less commonly used than nigrosin in routine bacterial smear preparations. It will give you a pink or reddish background rather than the dark grey/black background that you get with nigrosin.

In TEM negative staining you use various reagents. These are electron dense reagents such as uranyl acetate and phosphotungstic acid which you use to produce contrast in the TEM.

## Materials Required for Negative Staining

- Clean, grease-free glass slides

- Bacterial culture

- Nigrosin stain or India ink

- Sterile inoculating loop

- Sterile distilled water

- Dropper or Pasteur pipette

- Spreader slide

- Bunsen burner or microincinerator

- Light microscope

- Immersion oil

## Procedure of Negative Staining

![Two negative staining workflows showing preparation of a nigrosin bacterial smear with a 45-degree spreader slide and an India ink wet mount beneath a coverslip.](https://biologynotesonline.com/wp-content/uploads/2025/12/negative-staining-procedure-1024x437.webp)Nigrosin negative staining involves spreading a thin bacterial smear and allowing it to air-dry, while an India ink wet mount is examined beneath a coverslip without drying.

### Negative Staining Using Nigrosin (Spreader-Slide Method)

The negative staining procedure using nigrosin requires a bacterial culture, nigrosin stain, two clean glass slides, a sterile inoculating loop and a light microscope.

The steps of negative staining are given below:

- Take a clean, dry and grease-free microscope slide.

- Place a small drop of nigrosin near one end of the slide.

- Using a sterile inoculating loop, take a small amount of bacterial culture and mix it gently with the nigrosin. If you're using a solid culture, a small drop of sterile water may be used to suspend the bacteria, depending on the laboratory protocol.

- Hold a second clean slide at about 45° to the first slide. Bring its edge into contact with the bacterial stain mixture and allow the liquid to spread along the edge.

- Push the second slide smoothly across the first slide to prepare a thin and evenly spread smear.

- Allow the smear to air dry completely. Don't heat-fix the slide or wash off the stain.

- Examine the dried smear under the light microscope. Start by locating a suitable thin area and then use the required objective (usually the 100× [oil immersion objective](https://biologynotesonline.com/oil-immersion-technique-procedure/) for bacterial examination).

The bacterial cells appear clear against the dark background. The amount of stain, drying conditions and handling of the slides should follow the laboratory protocol being used.

### Negative Staining Using India Ink (Wet-Mount Method)

India ink is also used for negative staining, particularly for observing the capsules of microorganisms such as Cryptococcus neoformans. Unlike the nigrosin smear method, this preparation is examined as a wet mount.

The procedure is given below:

- Take a clean microscope slide and place a small amount of the appropriate specimen on it, such as an encapsulated yeast suspension.

- Add a drop of India ink to the specimen and mix them gently.

- Place a clean coverslip over the mixture, making sure that no air bubbles are trapped beneath it.

- Examine the preparation immediately under the light microscope using the appropriate objectives. The capsule appears as a clear halo around the cell against the dark background.

The India ink preparation is examined while it's still wet. It doesn't require spreading with a second slide, air drying or heat fixation.

Clinical specimens, including cerebrospinal fluid (CSF), must be handled and examined according to validated laboratory and diagnostic protocols.

## Results and Interpretation of Negative Staining

### Negative Staining of Bacteria

![Three illustrated microscope fields show pale rod-shaped bacteria, cocci in chains, and cocci in clusters against dark nigrosin-stained backgrounds.](https://biologynotesonline.com/wp-content/uploads/2025/12/negative-staining-bacterial-results-1024x576.webp)Three illustrated microscope fields show pale rod-shaped bacteria, cocci in chains, and cocci in clusters against dark nigrosin-stained backgrounds.

In a properly prepared negative stain slide the background is dark grey to black and the cells are clear/very lightly stained. You can see the cell shape against the stained background. The colour of the background will of course depend on the stain used. Nigrosin and India ink will give a dark background, some other acidic dyes will give a red or pink background.

The bacteria can be seen in a variety of shapes ([cocci, bacilli](https://biologynotesonline.com/morphology-of-bacteria-sizes-shapes-arrangements-examples-with-images/), curved or spiral) and in a variety of arrangements (single, pairs, chains, clusters). They should be well separated so that you can see both their shape and their arrangement.

For example E. coli in a nigrosin negative stain are small clear rods against a dark background. They are normally about 1-2µm long and 0.5-1µm wide (although the size can vary with the growth conditions). The size can be roughly assessed from the microscope appearance but actual size requires a [calibrated micrometer](https://biologynotesonline.com/ocular-micrometer-principle-parts-procedure-applications/) or scale bar.

The background should be fairly even with a thin distribution of stain. If you go overboard with the stain the cells can become quite difficult to see. Uneven spreading will result in dark patches and streaks on the slide. Cracking and other drying artefacts can also occur in the stained area, particularly in the thicker parts of the smear.

### Negative Staining of Encapsulated Microorganisms

In encapsulated micro-organisms the capsule appears as a clear halo around the cell. The stain stains the background but doesn't enter the capsule. There is therefore a clear space between the cell and the dark background.

When you use India ink to look at Cryptococcus neoformans the yeast cells are round or oval with a clear capsule. Some of the cells will be budding. The cell body is in the halo and the ink gives a dark background.

The size of the clear halo will vary from cell to cell, depending on the thickness of the capsule and the preparation. A good wet mount will show the cell body and the capsule boundary as distinct entities.

A halo is not necessarily indicative of a microbial capsule. Air bubbles, cell debris and other cells can all give a similar appearance. Uneven ink distribution can also be a factor. A halo alone does not confirm the presence of Cryptococcus and a negative India ink result does not rule out cryptococcal infection.

![Encapsulated Cryptococcus yeast cells appear with clear halos against an India ink background, with an enlarged view identifying the yeast body and capsule.](https://biologynotesonline.com/wp-content/uploads/2025/12/india-ink-negative-staining-cryptococcus-1024x524.webp)Encapsulated Cryptococcus yeast cells appear with clear halos against an India ink background, with an enlarged view identifying the yeast body and capsule.

## Applications of Negative Staining

The following are the applications of negative staining:

- Negative staining is used to observe the shape and size of bacterial cells without heat fixation.

- The arrangement of bacteria can also be examined, whether they are present singly, in pairs, chains or clusters.

- Delicate bacterial cells that may shrink or become distorted during heat fixation can be examined using negative staining.

- [Bacterial capsules](https://biologynotesonline.com/capsule-staining/) can be seen as clear halos around the cells against a dark background.

- India ink is used to examine encapsulated yeasts such as Cryptococcus neoformans. The capsule appears as a clear space around the yeast cell.

- Thin bacteria such as spirochetes can be observed by negative staining when sufficient contrast is obtained.

- Nigrosin is used in microbiology teaching laboratories to study the morphology of bacteria such as Escherichia coli.

- In transmission electron microscopy (TEM), heavy-metal negative stains are used to observe the structure and shape of [viruses and bacteriophages](https://biologynotesonline.com/negative-staining-of-viruses/).

- Proteins, protein aggregates, filaments and membrane vesicles are also examined by negative staining under TEM.

## Advantages of Negative Staining

The following are the advantages of negative staining:

- Negative staining is a simple and rapid method that requires only one stain.

- The bacterial cells are not heat-fixed, which helps prevent shrinkage and distortion caused by heating.

- It helps in observing the natural shape and approximate size of bacterial cells more accurately.

- The dark background makes the unstained bacterial cells easier to see under the microscope.

- Delicate bacteria that may get damaged during heat fixation can be examined without heating the smear.

- Bacteria can be observed singly or in pairs, chains and clusters.

- Bacterial capsules appear as clear halos around the cells. India ink can also be used to observe capsules in yeasts such as Cryptococcus neoformans.

- Thin bacteria such as spirochetes can be examined when the stain provides sufficient contrast.

- The smear doesn't need to be washed after staining. This helps prevent the loss of bacterial cells from the slide.

## Limitations of Negative Staining

The following are the limitations of negative staining:

- Negative staining is mainly used to observe the external shape and arrangement of bacterial cells. Internal cellular structures cannot be seen clearly.

- It cannot differentiate between Gram-positive and Gram-negative bacteria.

- The bacterial cells remain unstained, so their outlines may be difficult to observe if the background doesn't provide sufficient contrast.

- A thick smear or too much stain may cover the bacterial cells and make them difficult to distinguish under the microscope.

- Uneven spreading of the stain can produce dark patches and streaks on the slide.

- The smear is allowed to air dry without heat fixation. Drying may still cause some changes in the shape and arrangement of bacterial cells.

- Negative staining cannot determine whether the bacterial cells are living or dead.

- The unfixed bacterial cells may be removed or disturbed during handling of the slide.

- In India ink preparations, air bubbles and other particles may appear as clear halos and can be mistaken for microbial capsules.

- India ink staining may fail to detect encapsulated yeasts when very few organisms are present or the capsule is not clearly visible. A negative result doesn't rule out cryptococcal infection.

## Precautions of Negative Staining

The following are the precautions of negative staining:

- Use a clean, dry and grease-free glass slide for preparing the bacterial smear.

- Take a small amount of bacterial culture. Too many bacterial cells may form clumps and make observation difficult.

- The inoculating loop should be sterile and allowed to cool before taking the bacterial culture.

- Use a suitable amount of nigrosin stain. Excess stain may produce a thick and dark background.

- Spread the bacterial suspension gently with a second clean slide to prepare a thin and evenly spread smear.

- Allow the smear to air dry completely. Do not heat-fix the slide, as heating may cause shrinkage and distortion of bacterial cells.

- Do not wash or blot the smear after staining. The unfixed bacterial cells and stain may be removed from the slide.

- Select a thin area of the smear for microscopic examination. Avoid areas with excessive stain, dark patches or cracks formed during drying.

- Clean the oil immersion objective with lens paper after examination, as nigrosin may stick to the objective lens.

- In India ink wet-mount preparations, place the coverslip gently to avoid air bubbles. The preparation should be examined while it is still wet.

- Handle the unfixed bacterial smear carefully, as the organisms may remain viable. Used slides and contaminated materials should be disinfected or discarded according to the laboratory biosafety protocol.

## Difference Between Positive and Negative Staining

![Side-by-side microscope fields compare violet-stained bacterial rods on a pale background with unstained rods on a dark negative-staining background.](https://biologynotesonline.com/wp-content/uploads/2025/12/positive-vs-negative-staining-1024x683.webp)Side-by-side microscope fields compare violet-stained bacterial rods on a pale background with unstained rods on a dark negative-staining background.

Positive staining is used to stain the bacterial cells, while negative staining stains the background surrounding the cells. The differences between positive and negative staining are given below:

Positive StainingNegative StainingThe bacterial cells are stained directly.The background is stained while the bacterial cells remain clear or lightly coloured.Basic dyes such as crystal violet, methylene blue and safranin are commonly used.Acidic dyes such as nigrosin and Congo red are used. India ink is also used for negative staining.Basic dyes have positively charged chromophores which bind to the generally negatively charged bacterial surface.Acidic dyes have negatively charged chromophores which don't readily bind to the bacterial cells. India ink contains fine pigment particles that remain around the cells.The bacterial cells appear coloured against a clear or lightly coloured background.The cells appear clear against a dark or coloured background.Heat fixation is commonly done before staining in routine bacterial smear methods.Heat fixation is normally not done, as it may cause shrinkage and distortion of bacterial cells.The stain is applied to the smear and excess stain is usually washed off after staining.In the nigrosin method, the bacterial culture is mixed with the stain, spread on a slide and allowed to air dry without washing.It is used to observe the shape, size and arrangement of bacteria.It is used to examine bacterial shape, approximate size, arrangement and capsules.Positive staining may use a single dye or more than one dye, depending on the method.Negative staining commonly uses one staining reagent, such as nigrosin or India ink.

[Simple staining](https://biologynotesonline.com/simple-staining-procedure-principle-result/) refers to the use of a single staining reagent. It is commonly performed with a basic dye that stains the bacterial cells, but a negative staining method using one reagent may also be considered a simple stain.

Positive and negative staining should not be confused with Gram-positive and Gram-negative bacteria. [Gram staining](https://biologynotesonline.com/gram-staining-principle-procedure-results-uses/) is a separate differential staining method used to distinguish bacteria according to their Gram-staining reaction.

## Negative Staining in Electron Microscopy

![TEM schematic showing a biological particle on a carbon support film surrounded by electron-dense negative stain, with increased electron scattering in stained regions.](https://biologynotesonline.com/wp-content/uploads/2025/12/negative-staining-transmission-electron-microscopy-1024x524.webp)In TEM negative staining, heavy-metal-rich regions scatter more electrons and appear darker than the relatively unstained biological particles. Staining and drying can also introduce flattening or other preparation artifacts.

Negative staining in TEM is used to look at small biological particles such as viruses, protein complexes, vesicles and isolated cellular structures. The specimens are placed on a thin support film (typically carbon) which is attached to a metal grid.

Uranyl acetate and phosphotungstic acid are the stains of choice as negative stains in TEM. They are heavy atom stains which scatter the electrons more than the biological material itself. The stain will essentially remain around the particles and will also enter into the spaces that exist between the surface structures of the particles.

The excess stain is removed and the grid allowed to air dry. Under the EM the areas with more stain scatter more electrons and so appear dark. The biologicals appear quite light against this dark background and their shape and some of their surface detail can be seen.

Negative staining in TEM is quite different from bacterial negative staining under the light microscope. In the case of bacterial staining, the acidic dyes (e.g. nigrosin) won't readily adhere to the negatively charged bacterial cell surface. They just sit around the cells and stain the background. In TEM the contrast is due to the scattering of electrons by the heavy metal stain in the background rather than the colour of the stain.

In conventional TEM negative staining the specimen loses water as it dries and this can lead to some of the biological particles shrinking, collapsing or flattening. Membrane vesicles for instance can end up cup shaped as a result of the changes that occur during drying.

Cryo-EM uses specimens that are rapidly frozen in vitreous ice (i.e. without the usual heavy metal stain). The biological particles are kept in a hydrated state which helps to preserve their structure (as opposed to the drying out that occurs in negative staining). The downside of unstained specimens is that they generally give very low contrast in the original EM images.

## References

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- Brenner, S., & Horne, R. W. (1959). A negative staining method for high resolution electron microscopy of viruses. Biochimica et Biophysica Acta, 34, 103–110. [https://doi.org/10.1016/0006-3002(59)90237-9](https://doi.org/10.1016/0006-3002(59)90237-9)

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