Unit 1 Chapter 2

Question 1

• Explain the difference between simple and differential stains

Answer 1

Live samples are observed as a wet mount where the sample is suspended in liquid

Samples that require staining generally require attachment to the glass slide by fixation by heat or chemicals
Type of stains:
Acidic – chromophore has (-) charge
Basic – chromophore has (+) charge
Positive – cells are stained
Negative – background is stained, cells appear clear
Simple – only one dye (chromophore)
Differential - multiple dyes are used to differentiate organisms

Question 2

• Remember the procedure for the Gram Stain, and the purpose of each reagent

Answer 2

crystal violet - primary stain added to specimen smear stains cells purple or blue

Iodine - makes dyes less soluble so it adheres to cell walls cells remain purple or blue

Alcohol - decolorizer washes away stain from gram-negative cell walls gram + cells stay purple or blue gram - cells become colorless

Safranin - counterstain allows dye adherence to gram-negative cells Gram + cells remain purple or blue Gram - cells appear pink or red

Question 3

clinical purpose ( gram stain )

Answer 3

an effective method to distinguish between bacteria with different types of cell walls,

Question 4

Clinical purpose ( endospore stain )

Answer 4

uses two stains to differentiate endospores from the rest of the cell.

The Schaeffer-Fulton method (the most commonly used endospore-staining technique) uses heat to push the primary stain (malachite green) into the endospore. Washing with water decolorizes the cell, but the endospore retains the green stain. The cell is then counterstained pink with safranin. The resulting image reveals the shape and location of endospores if they are present. The green endospores will appear either within the pink vegetative cells or as separate from the pink cells altogether. If no endospores are present, then only the pink vegetative cells will be visible (

Endospore-staining techniques are important for identifying Bacillus and Clostridium, two genera of endospore-producing bacteria that contain clinically significant species.

Question 5

Clinical Purpose ( acid-fast stain

Answer 5

is able to differentiate two types of gram-positive cells: those that have waxy mycolic acids in their cell walls, and those that do not.

Two different methods for acid-fast staining are the Ziehl-Neelsen technique and the Kinyoun technique. Both use carbolfuchsin as the primary stain. The waxy, acid-fast cells retain the carbolfuchsin even after a decolorizing agent (an acid-alcohol solution) is applied. A secondary counterstain, methylene blue, is then applied, which renders non–acid-fast cells blue.

Question 6

Clinical Purpose ( negative capsule staining )

Answer 6

Certain bacteria and yeasts have a protective outer structure called a capsule.

Capsules do not absorb most basic dyes; therefore, a negative staining technique (staining around the cells) is typically used for capsule staining.

Question 7

Clinical Purpose ( flagella stain)

Answer 7

Flagella are tail-like cellular structures used for locomotion by some bacteria, archaea, and eukaryotes. Because they are so thin, flagella typically cannot be seen under a light microscope without a specialized flagella staining technique. Flagella staining thickens the flagella by first applying mordant (generally tannic acid, but sometimes potassium alum), which coats the flagella; then the specimen is stained with pararosaniline (most commonly) or basic fuchsin