Principles of Basic Staining Staining enhances the contrast of a cell and
helps one to better visualize the specimen and see various structures.
But what is a stain?
A stain is a solution, consisting of a color agent or dye known as a chromophore, and a
solvent, a solution in which the dye is dissolved.
The color agent, or chromophore, in a stain is a charged ion, and the charge of the chromophore
dictates the type of stain.
In a basic stain, the chromophore is a positively charge ion.
The positive charge of the dye will be attracted to the negative charge of a cell, and the
chromophore will pass through the cell's membrane, staining the specimen.
On the other hand, in an acidic stain, the chromophore is negatively charged.
The negative charge of the cell will prevent the dye from entering the specimen, thus the
chromophore remains outside the specimen and stains the background.
There are three main categories for staining: Simple, Differential and Special stains.
In a simple stain, only one stain is used, the process has few steps and doesn't require
much time; therefore, it's "simple" compared to other staining methods.
There are two types of simple staining techniques, Direct stain and Negative Stain.
In a direct stain, a basic stain is used.
Remember from our previous slides, a basic stain has a positively charged chromophore,
thus the specimen will stained in a direct stain.
In order to perform a direct stain, the specimen must first be fixed to a slide, so it will
remain on the slide during the staining process.
A direct stain is also referred to as a positive stain.
Methylene blue is a commonly used direct stain.
On the other hand, a negative stain uses an acidic type stain, such as Nigrosin.
Recall that acidic stains have negatively charged chromophores, thus in a negative stain
the background in colored, not the specimen.
Negative stains are often used for delicate specimens due to the fact you do not have
to fix the specimen to the slide in this staining procedure.
Here are examples of simple stains.
Notice in the negative stain image the background has retained the dark color of the stain,
whereas, the bacteria is still lighter and unstained.
In the direct stains, the specimens have picked up the stain and become colored, the background
is lighter and unstained.
Differential staining techniques require two or more stains, and have several steps to
complete.
Differential staining allows one to differentiate between specific cell characteristics.
In bacteria, differential stains are commonly used to determine the cell wall structure.
Two commonly used differential staining processes for bacteria are Gram's stain and Acid Fast
stain, both are used for identifying cell wall characteristics.
Here are examples of differential stains.
The first slide demonstrates a gram stain.
On the slide are two species of bacteria, one of which is a gram positive and stains
dark purple, the other a gram-negative which stained pink.
In the second image, a giant cell within the lungs contains numerous acid fast bacteria.
You can see the bacterial cells as they are stained pink against the blue
stain of the lung tissue.