Portage Microbiology Lab Exam 3
List the four main steps used to prepare a DRY mount, and indicate which step is
optional. –
1 - Clean slide2 - Circle area on slide for specimen placement (OPTIONAL)3 - Apply
organism to slide4 - Air dry at room temperature
Why is it important to first clean your slide before applying your sample? –
You must first remove any unwanted contaminants from the slide otherwise it may be
difficult to distinguish between the pathogen of interest and a contaminant.
You wish to study the motility of a particular bacterium. What type of mounting
technique would you use? Explain your answer. –
A wet mount technique is the best technique to use when studying motility of an
organism because the sample remains viable (heat fixing kills the sample).
What dye do Gram-positive bacteria primarily retain? –
crystal violet
Why are Gram-positive bacteria able to retain the crystal violet dye? –
They contain a thick peptidoglycan layer in their cell wall that readily retains the dye.
Identify the Gram status (positive or negative) and shape of the bacteria pictured below.
–
Gram-negative; Bacillus (rod)
When might you want to utilize a negative stain technique in the lab? What is the name
of the primary dye used in this technique? –
A negative stain is used to identify organisms with an opaque structure. Nigrosin dye.
Why do bacteria repel the dye nigrosin? –
Nigrosin is a negatively charged dye. The membranes of most cells are also negatively
charged. The membrane will repel the dye not allowing it to be absorbed.
What is one disadvantage of heat fixing a sample? –
The heat fixing procedure kills the specimen. This also prevents any observations on
motility and enzymatic properties.
What is the proper way to dispose of all materials used during the lab? –
All materials must be place in a biohazardous waste bag and placed into an autoclave
for sterilization.
What are the Gram status, shape and identification of organism #2 from the Gram stain
procedure? –
List the four main steps used to prepare a DRY mount, and indicate which step is
optional. –
1 - Clean slide2 - Circle area on slide for specimen placement (OPTIONAL)3 - Apply
organism to slide4 - Air dry at room temperature
Why is it important to first clean your slide before applying your sample? –
You must first remove any unwanted contaminants from the slide otherwise it may be
difficult to distinguish between the pathogen of interest and a contaminant.
You wish to study the motility of a particular bacterium. What type of mounting
technique would you use? Explain your answer. –
A wet mount technique is the best technique to use when studying motility of an
organism because the sample remains viable (heat fixing kills the sample).
What dye do Gram-positive bacteria primarily retain? –
crystal violet
Why are Gram-positive bacteria able to retain the crystal violet dye? –
They contain a thick peptidoglycan layer in their cell wall that readily retains the dye.
Identify the Gram status (positive or negative) and shape of the bacteria pictured below.
–
Gram-negative; Bacillus (rod)
When might you want to utilize a negative stain technique in the lab? What is the name
of the primary dye used in this technique? –
A negative stain is used to identify organisms with an opaque structure. Nigrosin dye.
Why do bacteria repel the dye nigrosin? –
Nigrosin is a negatively charged dye. The membranes of most cells are also negatively
charged. The membrane will repel the dye not allowing it to be absorbed.
What is one disadvantage of heat fixing a sample? –
The heat fixing procedure kills the specimen. This also prevents any observations on
motility and enzymatic properties.
What is the proper way to dispose of all materials used during the lab? –
All materials must be place in a biohazardous waste bag and placed into an autoclave
for sterilization.
What are the Gram status, shape and identification of organism #2 from the Gram stain
procedure? –
