Lab Quiz 1 Flashcards
What shape is bacillus megaterium
rod
what shape is staphylococcus epidermidis
round
Of what value is a simple stain?
to determine cell morphology, size, and arrangement
What is the purpose of heat-fixing the smear?
it denatures bacterial enzymes, preventing them from digesting cell parts. it also preserves the microbes
Another method of fixing smears is to use methanol instead of heat. How does alcohol chemically fix the bacteria?
the alcohol dehydrates the cell, causing them to denature and to be preserved onto the slide
In heat-fixing, what would happen if too much heat were applied?
cells will be distorted and unusable
Methylene blue can be prepared as a basic stain or an acidic stain. How would the pH of the stain affect the staining of bacteria?
the pH of the stain affects characteristics and transformation of bacteria
Can dyes other than methylene blue be used for direct staining?
Yes. crystal violet is catonic, and can directly stain positively charged microbes
Bacteria can be seen without staining. Why, then, was Koch’s recommendation of fixing and staining important for the discovery of the bacterial causes of diseases?
How bacteria stain is very important in medical diagnostics and treatment with the correct antibiotic. Also, staining bacteria can make small characteristics in the bacteria visible that otherwise would not be.
Quality control staff in a sterilization unit of a hospital used a simple stain to determine whether bacteria were present in sterilized materials. A simple stain of sterile saline used for respiratory therapy revealed the presence of bacteria. Is the saline contaminated?
It is possible. Negative controls and proper aseptic technique are needed to ensure the dye itself and immersion oil are not contaminated. These controls will help narrow down the source of the bacteria. However, staining does not show whether or not the cells were alive.
What color will a gram-negative cell stain?
red
What color will a gram-positive cell stain?
blue
What shape is bacillus subtilis?
rods
what shape is escherichia coli?
rods
How does staphylococcus epidermidis gram stain?
gram positive (blue)
How does bacillus subtilis gram stain?
gram positive (blue)
How does escherichia coli gram stian?
gram negative (red)
Why do gram-positive cells more than 24 hours old stain gram-negative?
The cells can no longer hold the primary stain.
Can iodine be added before the primary stain in a gram stain?
No, it will prevent the crystal violet from washing.
List the steps of the gram staining procedure in order (omit washings), followed by the color of gram-positive cells and gram-negative cells after each step.
STEP: GRAM+/GRAM-
- crystal violet: blue/blue
- iodine: blue/blue
- ethanol or acetone: blue/clear
- safranin: blue/red
Which step can you omit without affecting determination of the gram reaction?
safranin
Suppose you performed a gram stain on a sample from a pure culture of bacteria and observed a field of red and purple cocci. Adjacent cells were not always the same color. What would you conclude?
The sample is old and some of the cells are losing their ability to hold the primary stain.
Suppose you are viewing a gram-stained field of red rods and purple cocci through the microscope. What do you conclude?
the sample has a mixed culture
Human cells can be stained with crystal violet and safranin, so why can’t human cells be gram stained?
the only cells that stain gram positive are those that have a thick cell wall, so the primary would be easily removed by alcohol. safranin would bind to remaining structures with a negative charge.
