Microbiology Lab Exam Study Guide
What color do Gram-Positive bacteria stain? Correct Ans-Purple
What color do Gram-Negative bacteria stain? Correct Ans-Pink
What are the 4 steps of a gram reaction? Correct Ans-Primary stain crystal violet, mordant
iodine, decolorizer alcohol-acetone, and counterstain safranin.
What are some ways you could mess up a gram stain? And how will you know you messed up?
Correct 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
Look over cell shape and arrangement (i.e. cocci and strepto = Streptococcus) Correct Ans-
Look over colony morphology characteristics Correct Ans-
, Which grows better on MSA (Mannitol Salt Agar) staphylococcus or enterococcus? Why?
Correct 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
What is in BEA agar that makes it selective? Correct Ans-The Bile salts in the BEA are what
make it selective
What is in BEA agar that makes it Differential Correct Ans-The Esculin Azide in the agar is
what makes it differential
How can you tell if an enterococcus species is growing on a BEA plate? Correct Ans-The
media will turn dark brown
Why do we use EMB agar? Correct Ans-It allows us to selectively culture gram negative
bacteria and differentiate based on lactose fermentation.
Why is E. coli green on EMB? Correct Ans-Because it is fermenting the lactose in the agar
and is stained by the combined eosin Y and methylene blue dyes.
What species of bacteria does a catalase test differentiate between? Correct Ans-Between
staphylococcus/micrococcus and streptococcus/enterococcus bacteria
Which species is catalase negative? Correct Ans-Streptococcus/enterococcus species
What color do Gram-Positive bacteria stain? Correct Ans-Purple
What color do Gram-Negative bacteria stain? Correct Ans-Pink
What are the 4 steps of a gram reaction? Correct Ans-Primary stain crystal violet, mordant
iodine, decolorizer alcohol-acetone, and counterstain safranin.
What are some ways you could mess up a gram stain? And how will you know you messed up?
Correct 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
Look over cell shape and arrangement (i.e. cocci and strepto = Streptococcus) Correct Ans-
Look over colony morphology characteristics Correct Ans-
, Which grows better on MSA (Mannitol Salt Agar) staphylococcus or enterococcus? Why?
Correct 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
What is in BEA agar that makes it selective? Correct Ans-The Bile salts in the BEA are what
make it selective
What is in BEA agar that makes it Differential Correct Ans-The Esculin Azide in the agar is
what makes it differential
How can you tell if an enterococcus species is growing on a BEA plate? Correct Ans-The
media will turn dark brown
Why do we use EMB agar? Correct Ans-It allows us to selectively culture gram negative
bacteria and differentiate based on lactose fermentation.
Why is E. coli green on EMB? Correct Ans-Because it is fermenting the lactose in the agar
and is stained by the combined eosin Y and methylene blue dyes.
What species of bacteria does a catalase test differentiate between? Correct Ans-Between
staphylococcus/micrococcus and streptococcus/enterococcus bacteria
Which species is catalase negative? Correct Ans-Streptococcus/enterococcus species
