Chapter 2 - 1.C - Stains Flashcards
What are the three categories of positive stains?
Simple, Differential, and Special stains.
What are dye types and their qualities?
Basic dyes are cationic and have a positive charge therefore they attract cell parts that are negatively charged.
Acidic dyes are anionic and have a negative charge therefore they tend to be repelled by cells. They are good for negative staining where the background is stained but not the cells.
Simple Stains
Require only a single dye.
They cause all cells in a smear to appear more or less the same color, regardless of type, but they can still reveal bacterial characteristics such as shape, size, and arrangement.
Differential stains
Use two differently colored dyes, called the primary dye and the counterstain, to distinguish between cell types or parts. Differential staining techniques tend to be more complex and sometimes require additional chemical reagents to produce the desired reaction.
Types of differential stains include the Gram stain, the acid-fast stain, and the endospore stain.
The Gram Stain
Used to make bacter in infectious specimens more visible. Consists of sequential applications of crystal violet (the primary dye), Gram’s iodine (The mordant), an alcohol rinse (The decolorizer), and a contrasting counterstain.
Bacteria that stain purple are called Gram-Positive, those that stain red are called Gram-Negative. Gram-variable organisms produce both pink and purple stained cells.
Acid-fast Stains
Like the gram stain this is an important diagnostic stain that distinguishes acid-fast bacteria (pink) from non-acid-fast bacteria (blue). (Fast in this context means “Resistant to”)
Originated as a specific method to detect mycobacterium tuberculosis in specimens. It was determined that these bacterial cells have a particularly impervious outer wall that holds tightly to dye even when washed with a solution containing acid or acid alcohol.
Often performed when a gram-variable result is seen in a specimen.
Endospore Stain
Similar to acid-fast stain in that a dye is forced by heat into resistant bodies called endospores. This stain is designed to distinguish between endospores and the cells that they come from (So-called vegetative cells). Of significance in medical microbiology are the gram-positive, endospores-forming members of the genus Bacillus (cause of anthrax) and Clostridium *cause of botulism and tetanus).
Special Stains
Used to emphasize certain cell parts that are not revealed by conventional staining methods.
Capsular Staining
A method of observing the microbial capsule, an unstructured protective layer surrounding the cells of some bacteria and fungi. Because the capsule repels most stains it is often negatively stained with India ink, or it may be demonstrated by special positive stains. IE: Cryptococcus, which causes a serious form of fungal meningitis in AIDS patients.
Flagellar Staining
A method of revealing flagella, the tiny, slender filaments used by bacteria for locomotion. Because the width of bacterial flagella lies beyond the resolving power of the light microscope, in order to be seen they must be enlarged by depositing a coat on the outside of the filament and then staining it. Their presence, number, and arrangement on a cell are useful for identification of the bacteria.
