Activity 4 Flashcards
(24 cards)
Bacteria have almost the same refractive index as water. This means when you try to
view them using a microscope they appear as faint, gray shapes and are difficult to visualize.
one method for making microbial cells easier to visualize.
Staining
use only one dye that stains the cell wall of bacteria much like dying eggs at Easter.
simple stains
use two or more stains and categorize cells into groups.
differential stains
The most common differential stain used in microbiology is the ______.
Gram stain
What are the four reagents used in the Gram stain procedure?
Crystal violet
Iodine
decolorizer (acetone/alcohol)
safranin
What is the primary stain in Gram staining?
Crystal violet
Question: What reagent acts as a mordant in Gram staining?
Gram’s iodine
What is the function of Gram’s iodine in Gram staining?
It reacts with crystal violet to form a large crystal complex that is not easily washed out of the bacterial cell wall.
What is the decolorizer used in Gram staining?
A solution of acetone and alcohol (95% ethanol)
Why does decolorizer affect Gram-negative bacteria but not Gram-positive bacteria?
Gram-negative bacteria have lipopolysaccharides in their cell walls that dissolve in the decolorizer, allowing the primary stain to be washed out.
Some bacteria have relatively thick cell walls composed primarily of a carbohydrate known as _____.
peptidoglycan
Other bacterial cells have thinner cell walls composed of _____ and _____.
peptidoglycan and lipopolysaccharides.
Peptidoglycan is not soluble in organic solvents such as alcohol or acetone, but lipopolysaccharides are nonpolar and will dissolve in nonpolar organic solvents.
What is the counterstain used in Gram staining?
Safranin
How do Gram-positive bacteria appear after Gram staining?
Blue or purple
How do Gram-negative bacteria appear after Gram staining?
Pink or red
What is the name of the toxin released by Gram-negative bacteria when they die?
Endotoxin
What are round, purple cells classified as in Gram staining?
Gram-positive cocci
What are rod-shaped, purple cells classified as in Gram staining?
Gram-positive bacilli
a differential staining that is used to distinguish between vegetative cells and endospores.
malachite green staining procedure
Typical gram-positive bacteria are those staphylococci that produce boils
typical gram-negative bacteria are the bacilli that cause whooping cough
typical gram-variable bacteria are the bacilli that cause tuberculosis.
What is the most critical step in the Gram stain procedure?
The decolorization step
Why is a counterstain necessary in Gram staining?
It stains Gram-negative bacteria, which lost the primary stain during decolorization, making them visible.
