Most bacteria are quite colourless
and transparent and have a refractive index similar to that of the aqueous fluids in which they are
suspended. Unless the diaphragm is carefully adjusted usually there is considerable difficulty in
bringing the organisms into focus. Small size of bacteria can
be seen with the ordinary light microscope unless the organisms are stained.Some staining techniques, such as the Gram and Ziel Neelsen stains,
although of great diagnostic value because of their differential staining properties for specific
bacteria, reveal little internal structure. Other such as Feulgen stain for nuclear bodies,
demonstrates specific structure.
HANGING DROP TECHNIQUE
The techniques employed are meant for microscopic observation of living bacteria. The motility
The study of bacterial morphology is performed in two ways:
1. Observing unstained cells live by hanging drop preparation.
2. Observing dead cells by making use of chemical nature of their unicellular, body.
This is
achieved by staining.
Hanging drop technique enables viewing of size shape, arrangement and motility of live
microorganisms in fluid media. It requires the use of special ground slides. In this technique a
loopful of bacterial suspension is placed in the centre of a cover slip. In the four corners tiny
droplets of mineral oil are placed. The hollow ground slide is placed over the cover slip with the
depression side down and the slide is inverted quickly so that the water cannot run off to one
side. However, the lack of contrast yields limited though valuable information.The
method you use will depend on which one is most suitable for the situation at hand.
Hanging Drop Preparation or Motility Test
1. Apply vaseline around the depression of the hanging-drop slide.
2. Using the inoculation loop, aseptically transfer one drop of the culture to the centre of a
clean cover slip.
3. Invert the hanging-drop slide and centre its well over the drop of the culture, Press down
on the edge of the cover slip so that the vaseline makes a firm seal.
4. Quickly and carefully turn the slide right side up so as to suspend the hanging drop in the
well. Don’t let the drop fall or touch the bottom of the well.
5. To examine, first locate its edge in centre of the microscopic field with low power
objective and markedlv lower the light. The edge will be visible, as a bright wavy like
against a dark background. Now the slide can be focused under oil immersion .
When working with pathogenic microorganisms such as the typhoid bacillus, it is too
dangerous to attempt to determine motility with slide techniques. A much safer method is to
culture the organisms in a special medium that can demonstrate the presence of motility. The
procedure is to inoculate a tube of semisolid or SIM medium that can demonstrate the presence
of motility. Both media have a very soft consistency that allows motile bacteria to migrate
readily through them causing cloudiness.
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| PROCEDURE |
MICROBIAL STAINING
It is a chemical or a physical union between the dye and like component of a cell. If it is a
chemical reaction a new compound is formed and a simple washing with water does not liberate
the bound dye but if purely physical it is easy to decolorize such stained organism. Usually it is a
combination of chemical and physical reactions.
The main advantages of staining are
(i) Provides contrast between microorganisms and their backgrounds, permitting
differentiation among various morphological types;
(ii) Permits study of internal structures of the bacterial cell, such as the cell wall, vacuoles or
nuclear bodies and other cellular structures;
(iii) and enables the bacteriologist to use higher magnifications.
Fixing
Before staining it is essential to fix the bacterial sample on to the slide. Smear is prepared
in the following way:
(i) With a wire loop place a small drop of the broth culture or a loop full of bacteria on a
clean slide.
(ii) Place a drop of water over it.
(iii) Spread the culture so as to form a thin film.
(iv) Allow slide to dry in the air or by holding it above a bunsen flame.
(v) Avoid excess heating.
The purpose of fixation is to kill the microorganisms, coagulate the protoplasm of the cell
and cause it to adhere to the slide.
SIMPLE STAINING
The use of a single stain to colour a bacterial organism is commonly referred as simple staining.
All these dyes work well on bacteria as they have colour bearing ions (chromatophores) and arepositively charged. The fact that bacteria are slightly negatively charged when the pH of the
surrounding is near neutrality and produces a pronounced attraction between these cationic
chromatophores and the organism so that the cell is stained.
Such dyes are classified as basic
dyes. Crystal violet and carbol fuschin are some other examples. Those dyes that have anionic chromatophores are called acidic dyes. Eosine
(sodium+
eosine-
) is such a dye. The anionic chromatophores, eosine-
, will not stain
bacteria because of the electrostatic repelling forces that are involved.
The staining times for most simple stains are relatively short, usually from 30 seconds to
2 minutes, depending on the affinity of the dye. After a smear has been stained for the required
time, it is washed off gently, blotted dry, and examined directly under oil immersion. Such a
slide is useful in basic morphology.
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| PROCEDURE |
NEGATIVE STAINING
A better way to observe bacteria for the first time is to prepare a slide by a process called
negative or background staining. This method consists of mixing the microorganisms in a small
amount of nigrosine or india ink and spreading the mixture over the surface of the slide
(nigrosine is far superior to india ink). Since these two pigments are not really bacterial stains,
they do not penetrate the microorganisms; instead they obliterate the background, leaving the
organisms transparent and visible in a darkened field. Although this technique has a limitation, it
can be useful for determining cell morphology and size. Since no heat is applied to the slide,
there is no shrinkage of the cell and consequently more accurate cell size determination result
than with some other methods. This method is also useful for studying spirochaetes that does not
stain readily with ordinary dyes.
Negative staining can be performed by one of the following methods. Somewhere between the too thick and too thin areas
will be an ideal spot to study the organisms.
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| PROCEDURE |
Differential Staining
Gram staining
Developed by Christian Gram in 1884; by using this procedure, bacteria are subdivided by their
reaction to this stain into those which retain it, termed Gram-positive, and those which are
decolourized, termed Gram-negative.
Gram staining requires 4 different solutions.
(i) A Basic Dye: Discussed previously.
(ii) A Mordant: It increases the affinity or attraction between the cell and the dye, e.g.
Iodine.
(iii) A Decolorising agent: It removes the dye from a stained cell, e.g., alcohol, acetone or
ether.
(iv) Counter stain: It is a basic dye of a different colour than the initial one, e.g., Safranin.
Stain Preparation
(i) Crystal Violet: Dissolve 2 g of crystal violet in 20 mL of 95% ethanol. Dissolve 0.8 g of
ammonium oxalate in 80 mL of distilled water. Mix these 2 solutions; stand for 24 h and
f1lter.
(ii) Gram's Iodine: Dissolve 2 g of potassium iodide and 1 g of iodine in 300 mL of distilled
water.
(iii) Safranin: Grind 0.25 g of Safranin in a mortar with 10 mL of 95% ethanol.
Principle
The differential response towards Gram reaction is attributed to the difference in the cell wall of
these bacteria. Gram negative bacteria have cell wall generally thinner than those of gram
positive bacteria. Gram negative ones have higher lipid content than the gram positive bacteria.
During staining of gram positive bacteria, the alcohol treatment extracts the lipid, which results
in increased porosity or permeability of the cell wall. The CV-I (crystal violet iodine) complex
can be extracted and the gram negative organism is decolorized. These cells subsequently take
the colour of the safranin a Counter stain. The cell walls of gram positive bacteria, due to their
different composition (lower lipid content) become dehydrated during treatment with alcohol
presumably causes diminution in the pore diameter of the walls of peptioglycan and CV-I
complex is trapped in the wall following ethanol treatment. The pore size decreases, permeability
is reduced, and CV-I complex cannot be extracted. Hence these cells remain purple violet. While
in gram negative bacteria the amount of peptidoglycan is very low hence the cross linking is
reduced thus making space for crystal violet iodine complex to escape
Procedure
1. Prepare smear on a clean slide.
2. Stain with crystal violet for 30 seconds.
3. Rinse with water.
4. Flood the f1lm with Grams iodine and allow it to act for 30 sec.
5. Rinse with water.
6. Decolorise with 95% alcohol.
7. Rinse with water.
8. Counter stain with safranin for 20-30 sec.
9. Rinse with water and blot dry.
10. Examine under oil immersion objective.
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| PROCEDURE |
Acid fast stain
The Ziel-Nehlson method employs carbol fuchsin, acid alcohol and a blue or green counter stain.
Acid fastness is a phenomenon that is found in certain types of bacteria where they resist
the process of de-colorization that occurs when acid is used to wash a sample that contains these
bacteria. These bacteria also resist staining and it may require heat and concentrated staining to
colorize them. Once this colorization has taken place, it becomes difficult to decolorize using
acid, or stain them with another color unless the heat and concentration technique is used. This is
why the term given to them is 'acid fast' just like one would use the term 'color fast' for cloth that
does not leak color when washed (Figure 6.8).
The most common type of acid fast bacteria is mycobacteria. A strain of mycobacteria is
responsible for the disease tuberculosis. In diagnostic medicine, the acid fast test may be used on
a person in order to detect the existence of the bacteria that can cause tuberculosis. It is therefore
used as a diagnosis of the disease because other symptoms of tuberculosis are not useful as they
overlap with other conditions. Acid fast stain results can confirm the presence of the bacteria
known as mycobacteria tuberculosis which is the bacteria responsible for causing tuberculosis. In
laboratories where large number of sputum, gastric washings, urine, and other body fluid
samples are tested for pathogenic mycobacteria, fluorochrome acid fast staining is used in
conjunction with the Ziel-Nehlson method. The advantage of using a fluorescence method is that
fluorochrome stained slides can be scanned under lower magnification, while a Ziel-Nehlson
prepared slide must be examined under oil immersion (1000X magnification), fluorochrome
stained slides can be examined with 60X or 100X magnification.
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| PROCDURE |
Stain Preparation
(i) Carbol fuchsin
1. Basic fuchsin 58 g
2. Crystalline phenol 25 g
3. 95% alcohol 50 mL
4. Distilled water to 500 mL
Dissolve the fuchsin and phenol in alcohol over a warm water bath, and then add water.
Filter, before use.
(ii) 95% ethyl alcohol 970 mL
Concentrated HCl 30 mL
(iii) 0.5% methylene blue or malachite green in distilled water.
Procedure
Pour carbol fuchsin on the slide and heat carefully until steam rises. Stain for 3-5 min but
do not allow to dry. Wash well with water. Decolorize using acid alcohol for 10-20 s. changing
twice and counter stain for 2 min with methylene blue or malachite green. Acid fast organisms,
e.g. Mycobacterium tuberculosis and Mycobacterium leprae stain well
Endospore stain
Species of bacteria, belonging principally to the genera Bacillus and Clostridium produce
extremely heat resistant structures called endospores. In addition to being heat-resistant, they are
also resistant to many chemicals that destroy non-spore forming bacteria. This resistance to heat
and chemicals is due primarily to a thick, tough spore coat. They resist staining and, once stained
they resist decolorisation and counter staining. Several methods are available that employ heat to
provide stain penetration. However, the malachite green-Schaeffer and Fulton and Dorner
methods are commonly used by most bacteriologists. Thus endospore stains green but rest of the
cell or a cell without endospore stains light red
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| PROCEDURE |
(i) The malachite green -Schaeffer and Fulton Method
Procedure
1. Prepare a thin smear on a clean slide.
2. Place the slide on staining rack above boiling water.
3. Cover the smear with small pieces of paper towel, keep saturated with malachite green
(5% aqueous solution) and continue heating for 5 min.
4. Wash gently with water.
5. Counter stain with safranin for 30 seconds.
6. Wash with water and blot dry.
7. Examine under oil immersion objective
(ii)Dorner method
The Dorner method for staining endospores produces a red spore within a colourless
sporangium. Nigrosine is used to provide a dark background for contrast.The sporangium and
endospore are stained during boiling in step 3, however, the sporangium is decolourized by the
diffusion of safranin molecules into the nigrosine.
Capsule staining
Place a small drop of Indian ink on a clean slide. Mix into it a small loopful of bacterial culture.
Drop a cover glass and blot off excess ink. Examine.
For dry preparations mix one loopful of Indian ink with one loopful of suspension of
organism in 5% dextrose solutions at one end of a slide. Allow to dry and pour a. few drops of
methyl alcohol on it keep the slide over the flame to fix. Stain for a few sec. with 0.5.% aq.
solution of methyl violet. The capsule will appear as haloes in blue cell of bacterium under
microscope
Flagellar staining
(i) Mordant
20% tannic acid solution 3 parts
5% tartar emetic solution 2 parts
Distilled water 5 parts
Mix and redissolve the heavy precipitate by boiling.
(ii) Silver solution
Dilute a saturated solution of silver sulphate with an equal volume of
distilled water. Add 33% monoethylamine till the precipitate formed is redissolved. Solution can
be stored for long term, heat gently before using.
Procedure
1. Take a clean slide and transfer a loopful of bacterial suspension holding the slide
vertically. Allow the drop to run to the other end. Dry the film in air or in incubator.
2. Heat the mordant in a test tube to a boiling point and flood the slide with it wait for 2
minutes. Wash well in hot- water and finally in cold water. Allow to dry.
3. Cover the film with silver solution till thick portion of the film becomes dark to brown
and metallic sheen appears on the edge.
4. Wash with water and blot dry.
5. For preparing a permanent slide dip the latter in a weak solution of gold chloride for 1 or
2 hr. Wash and mount in canada balasam.
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