Some bacteria like Bacillus anthracis have a
well organized layer of material lying outside the cell wall referred to as a
capsule (or K antigen). What really differentiates a capsule from any other
material that may form on the surface of a cell wall of other bacteria is the
fact that capsules are well organized layers that cannot be easily washed off.
Typically, a capsule is composed of capsular polysaccharides (CPS) which are polysaccharides. They may also be composed of a number of other materials such as poly-D-glutamic acid. For some bacteria, capsules are very important in that they are a major virulence factor.
As such, they protect the bacteria from phagocytic actions of such cells as neutrophils allowing the bacteria to thrive. This is achieved due to the fact that the capsule is very smooth and has a negative charge that prevents attachments/adherence.
Some of the other significance of a capsule
Capsule stain is a type of differential stain
which involves the use of two stains; primary stain and the counterstain. Here,
it's worth noting that for most part, capsules are non-ionic.
As a result,
acidic and basic stains will often fail to adhere on to them. For this reason,
it becomes necessary to stain the background using an acidic stain while the
cell is stained using a basic stain.
Two of the most commonly used methods used in
capsule staining include:
India ink method
Anthony's stain method
India Ink Method
Place a drop of India ink
onto a clean glass slide
Using a sterile loop,
obtain a sample and place it onto the slide and mix it with the drop of India
Obtain another sterile
glass slide and having laid it at an angle on one end of the first slide,
spread out the drop into a film to have a thin layer of the smear-ink mixture
Allow the slide to stand
for about 5 minutes (air- dry)
Saturate the slide with
crystal violet for a minute and then tilt the slide to drain excess dye
Allow the slide to dry
View the slide using 100x
India ink is often used as the negative stain.
However, some of the other stains that can also be used include:
This is a negative staining technique that is
essentially used to identify the presence of capsules. Because of its acidic
nature, India ink (or Congo red, nigrosin) stains the background dark.
On the other hand, crystal violet is used for number of reasons including:
To act as a fixative
Increase penetration power
Stain the cells (being a
Decrease pH of smear
When viewed under the microscope, the background
will be dark as a result of India ink, bacteria cells will be purple having
taken the crystal violet dye while the capsule will be clear against a dark
background given that it takes no stain.
* Nigrosin is recommended to India ink because it
gives a more even background and spreads more easily.
Anthony's Stain Method
With Anthony’s stain method, the primary stain
used is crystal violet.
Some of the other requirements include:
A sample (36-48 hour culture
of capsulated bacteria)
Introduce a drop of crystal
violet on to a clean glass slide
Using a sterile loop, place
the sample on to the glass slide
Obtain another glass slide
and at an angle, spread the drop and sample to form a thin film
Allow the film to dry (air
dry) for about 6 minutes
Title the slide and rinse
with 20% copper sulfate solution
Allow the side to air dry
for about 3 minutes
Place the slide on to the
microscope stage and observe using oil immersion
This is a positive staining method used in
capsule staining. Here, the smear/sample is treated with a hypertonic solution
(copper sulfate) for the purposes of creating an ionic difference that
ultimately results in the diffusion of a stain towards the outer surface of the
After the slide is air dried, it becomes possible to observe the stain
that remained on the capsular layer. Here, one will see a dark violet color of
the cell and a light violet color of the capsule.
Unlike endospores, capsules are more delicate.
For this reason, using heat during the staining process should be avoided given
that capsules would be easily destroyed. Rinsing the slide with
water is also avoided in capsule staining because it would dislodge the capsule.
A drop of serum may be used on the smear to enhance the size of the capsule.
This helps make it more visible to the technician under the typical light
Given that this is a sensitive procedure, sterile technique should
be used when obtaining the sample and introducing it onto the slide. This
prevents any contamination and helps determine whether the sample really has
any bacteria capsules.
Significance of Capsule Stain
As previously mentioned, capsules are considered
to be the virulence factor which means that they enhance the ability of a
bacterial to cause a disease. With a capsule in place, the bacteria is
protected from such body defense cells as macrophages. Therefore, it becomes
easier for the bacteria to thrive and cause diseases. For this reason, capsule
stain is of great benefit in that it helps identify the presence of capsules
and thus allowing healthcare professionals to recommend the right treatment.
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