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2024/2025 MICROBIOLOGY LAB EXAM STUDY GUIDE WITH CORRECT ANSWERS
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2024/2025 MICROBIOLOGY LAB EXAM STUDY GUIDE WITH CORRECT ANSWERS
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2024/2025 MICROBIOLOGY LAB EXAMSTUDY GUIDE WITH CORRECT
ANSWERS
What color do Gram-Positive bacteria stain? - CORRECT ANSWERS- Purple
What color do Gram-Negative bacteria stain? - CORRECT ANSWERS- Pink
What are the 4 steps of a gram reaction? - CORRECT ANSWERS- Primary
stain crystal violet, mordant iodine, decolorizer alcohol-acetone, and
counterstain safranin.
What is in BEA agar that makes it selective? - CORRECT ANSWERS- The Bile
salts in the BEA are what make it selective
What is in BEA agar that makes it Differential - CORRECT ANSWERS- 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 ANSWERS- The media will turn dark brown
Why do we use EMB agar? - CORRECT ANSWERS- It allows us to selectively
culture gram negative bacteria and differentiate based on lactose
fermentation.
, Why is E. coli green on EMB? - CORRECT ANSWERS- Because it is fermenting
the lactose in the agar and is stained by the combined eosin Y and
methylene blue dyes.
What are some ways you could mess up a gram stain? And how will you
know you messed up? - CORRECT ANSWERS- 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 ANSWERS-
Look over colony morphology characteristics - CORRECT ANSWERS-
Which grows better on MSA (Mannitol Salt Agar) staphylococcus or
enterococcus? Why? - CORRECT ANSWERS- 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
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