2025-2026 SERIES
MICROBIOLOGY LAB EXAM STUDY GUIDE COMPLETE
QUESTIONS AND CORRECT ANSWERS | GRADED
AND THE LATEST SERIES
What color do Gram-Positive bacteria stain?
Ans: Purple
Question: What color do Gram-Negative bacteria stain?
Ans: Pink
Question: What are the 4 steps of a gram reaction?
Ans: Primary stain crystal violet, mordant iodine, decolorizer alcohol-acetone,
and counterstain safranin.
Question: What are some ways you could mess up a gram stain? And how will
you know you messed up?
Ans: a. Not heat fixing your slide - clear slide,
b. over decolorizing - slide has both pink and purple even though the culture was
pure, so gram positive bacteria will stain pink and purple
c. Under decolorizing - slide has both pink and purple even though the culture was
pure, so the gram negative bacteria will stain purple and pink
d. Overheating your slide or not letting your smear completely dry - bacteria will
burn off leaving your slide clear
e. Forgetting to counterstain - you will have gram positive bacteria that appear
purple and gram negative bacteria will appear colorless
, 2025-2026 SERIES
Question: Look over cell shape and arrangement (i.e. cocci and strepto =
Streptococcus)
Ans:
Question: Look over colony morphology characteristics
Ans:
Question: Which grows better on MSA (Mannitol Salt Agar) staphylococcus or
enterococcus? Why?
Ans: Staphylococcus bacteria grows better on MSA because it has a high salt
content that makes it very difficult for enterococcus bacteria to grow on it. Also
some staphylococcus species, like staphylococcus aureus, are able to ferment the
mannitol in the agar and change the pH of the agar as evidenced by a color change
from pink to yellow
Question: What is in BEA agar that makes it selective?
Ans: The Bile salts in the BEA are what make it selective
Question: What is in BEA agar that makes it Differential
Ans: The Esculin Azide in the agar is what makes it differential
Question: How can you tell if an enterococcus species is growing on a BEA
plate?
Ans: The media will turn dark brown
MICROBIOLOGY LAB EXAM STUDY GUIDE COMPLETE
QUESTIONS AND CORRECT ANSWERS | GRADED
AND THE LATEST SERIES
What color do Gram-Positive bacteria stain?
Ans: Purple
Question: What color do Gram-Negative bacteria stain?
Ans: Pink
Question: What are the 4 steps of a gram reaction?
Ans: Primary stain crystal violet, mordant iodine, decolorizer alcohol-acetone,
and counterstain safranin.
Question: What are some ways you could mess up a gram stain? And how will
you know you messed up?
Ans: a. Not heat fixing your slide - clear slide,
b. over decolorizing - slide has both pink and purple even though the culture was
pure, so gram positive bacteria will stain pink and purple
c. Under decolorizing - slide has both pink and purple even though the culture was
pure, so the gram negative bacteria will stain purple and pink
d. Overheating your slide or not letting your smear completely dry - bacteria will
burn off leaving your slide clear
e. Forgetting to counterstain - you will have gram positive bacteria that appear
purple and gram negative bacteria will appear colorless
, 2025-2026 SERIES
Question: Look over cell shape and arrangement (i.e. cocci and strepto =
Streptococcus)
Ans:
Question: Look over colony morphology characteristics
Ans:
Question: Which grows better on MSA (Mannitol Salt Agar) staphylococcus or
enterococcus? Why?
Ans: Staphylococcus bacteria grows better on MSA because it has a high salt
content that makes it very difficult for enterococcus bacteria to grow on it. Also
some staphylococcus species, like staphylococcus aureus, are able to ferment the
mannitol in the agar and change the pH of the agar as evidenced by a color change
from pink to yellow
Question: What is in BEA agar that makes it selective?
Ans: The Bile salts in the BEA are what make it selective
Question: What is in BEA agar that makes it Differential
Ans: The Esculin Azide in the agar is what makes it differential
Question: How can you tell if an enterococcus species is growing on a BEA
plate?
Ans: The media will turn dark brown
