Simple and Differential Staining

 STAINING METHODS

INTRODUCTION

Staining is technique used in microscopy to enhance contrast in the microscopic image. Stains and dyes are frequently used in biological tissues for viewing, often with the aid of different microscopes. Stains may be used to define and examine bulk tissues (highlighting, for example, muscle fibers or connective tissue), cell populations (classifying different blood cells, for instance), or organelles within individual cells. Bacteria have nearly the same refractive index as water, therefore, when they are observed under a microscope they are opaque or nearly invisible to the naked eye. Different types of staining methods are used to make the cells and their internal structures more visible under the light microscope. Microscopes are of little use unless the specimens for viewing are prepared properly. Microorganisms must be fixed & stained to increase visibility, accentuate specific morphological features, and preserve them for future use

STAIN

A stain is a substance that adheres to a cell, giving the cell color. The presence of color gives the cells significant contrast so they are much more visible. Different stains have different affinities for different organisms, or different parts of organisms. They are used to differentiate different types of organisms or to view specific parts of organisms

STAINING TECHNIQUES

Direct staining – The organism is stained and background is left unstained Negative staining – The background is stained and the organism is left unaltered Stains are classified as

*  Simple stain

*  Differential stain

Fixing Before staining it is essential to fix the bacterial sample on to the slide. Smear is prepared in the following way:

• With a wire loop place a small drop of the broth culture or a loop full of bacteria on a clean slide.
• Place a drop of water over it.
• Spread the culture so as to form a thin film.
• Allow slide to dry in the air or by holding it above a bunsen flame
• 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 staining process involves immersing the sample (before or after fixation and mounting) in dye solution, followed by rinsing and observation. Many dyes, however, require the use of a mordant, a chemical compound that reacts with the stain to form an insoluble, coloured precipitate. When excess dye solution is washed away, the mordant stain remains. Simple staining is one step method using only one dye. Basic dyes are used in direct stain and acidic dye is used in negative stain. Simple staining techniques is used to study the morphology better, to show the nature of the cellular contents of the exudates and also to study the intracellular location of the bacteria.

Commonly used stains are

• Methylene blue
•   Dilute carbol fuchsin
•   Polychrome methylene blue

SIMPLE STAINING PROCEDURE

When a single staining-reagent is used and all cells and their structures stain in the same manner, the procedure is called simple staining procedure. This procedure is of two types – positive and negative. In positive staining, the stain (e.g., methylene blue) is basic (cationic) having positive charge and attaches to the surface of object that is negatively charged. In negative staining, the stain (e.g., India ink, nigrosin) is acidic (anionic) having negative charge and is repelled by the object that is negatively charged, and thus fills the spaces between the objects resulting in indirect staining of the object.

                                               Procedure of simple staining

DIFFERENTIAL STAINING

Differential Stains use two or more stains and allow the cells to be categorized into various groups or types. Both the techniques allow the observation of cell morphology, or shape, but differential staining usually provides more information about the characteristics of the cell wall (Thickness). Gram staining (or Gram’s method) is an empirical method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative) based on the chemical and physical properties of their cell wall. The Gram stain is almost always the first step in the identification of a bacterial organism, While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, thus forming Gram variable and Gram indeterminate groups as well.

GRAM STAINING

Gram Staining is the common, important, and most used differential staining techniques in microbiology, which was introduced by Danish Bacteriologist Hans Christian Gram in 1884. This test differentiates the bacteria into Gram Positive and Gram Negative Bacteria, which helps in the classification and differentiations of microorganisms.

PRINCIPLE OF GRAM STAINING

When the bacteria is stained with primary stain Crystal Violet and fixed by the mordant, some of the bacteria are able to retain the primary stain and some are decolorized by alcohol. The cell walls of gram positive bacteria have a thick layer of protein-sugar complexes called peptidoglycan and lipid content is low. Decolorizing the cell causes this thick cell wall to dehydrate and shrink which closes the pores in the cell wall and prevents the stain from exiting the cell. So the ethanol cannot remove the Crystal Violet-Iodine complex that is bound to the thick layer of peptidoglycan of gram positive bacteria and appears blue or purple in colour. In case of gram negative bacteria, cell wall also takes up the CV-Iodine complex but due to the thin layer of peptidoglycan and thick outer layer which is formed of lipids, CV-Iodine complex gets washed off. When they are exposed to alcohol, decolorizer dissolves the lipids in the cell walls, which allows the crystal violet-iodine complex to leach out of the cells. Then when again stained with saffranin, they take the stain and appear red in color.

MATERIAL REQUIRED

Clean glass slides, inoculating loop, Bunsen burner, Bibulous paper ,Microscope ,Lens paper and lens cleaner, Immersion oil, Distilled water , 18 to 24 hour cultures of organisms

REAGENTS

1. Primary Stain – Crystal Violet

2. Mordant – Grams Iodine

3. Decolourizer – Ethyl Alcohol

 4. Secondary Stain – Saffranin

GRAM STAIN PROCEDURE

1. Place slide with heat fixed smear on staining tray.

2. Gently flood smear with crystal violet and let stand for 1 minute.

3. Tilt the slide slightly and gently rinse with tap water or distilled water using a wash bottle.

 4. Gently flood the smear with Gram’s iodine and let stand for 1 minute.

5. Tilt the slide slightly and gently rinse with tap water or distilled water using a wash bottle. The smear will appear as a purple circle on the slide.

6. Decolorize using 95% ethyl alcohol or acetone. Tilt the slide slightly and apply the alcohol drop by drop for 5 to 10 seconds until the alcohol runs almost clear. Be careful not to over-decolorize.

7. Immediately rinse with water.

8. Gently flood with saffranin to counter counter-stain and let stand for 45 seconds.

9. Tilt the slide slightly and gently rinse with tap water or distilled water using a wash bottle.

10. Blot dry the slide with bibulous paper.

11. View the smear using a light-microscope under oil-immersion

INTERPRETATION METHODS

Gram Positive: Blue/Purple Color

Gram Negative: Red Color

GRAM POSITIVE BACTERIA-Actinomyces, Bacillus, Clostridium, Corynebacterium, Enterococcus, Gardnerella, Lactobacillus, Listeria, Mycoplasma, Nocardia, Staphylococcus, Streptococcus,Streptomyces etc.

GRAM POSITIVE BACTERIA-Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas,Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella etc.