Microbiology Lab Exam Study Guide
Questions and Answers 100% Pass
What color do Gram-Positive bacteria stain? - ✔✔Purple
What color do Gram-Negative bacteria stain? - ✔✔Pink
What are the 4 steps of a gram reaction? - ✔✔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? - ✔✔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
1
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, 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) - ✔✔
Look over colony morphology characteristics - ✔✔
Which grows better on MSA (Mannitol Salt Agar) staphylococcus or
enterococcus? Why? - ✔✔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? - ✔✔The Bile salts in the BEA
are what make it selective
2
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Questions and Answers 100% Pass
What color do Gram-Positive bacteria stain? - ✔✔Purple
What color do Gram-Negative bacteria stain? - ✔✔Pink
What are the 4 steps of a gram reaction? - ✔✔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? - ✔✔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
1
100% Pass Guarantee Katelyn Whitman, All Rights
, 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) - ✔✔
Look over colony morphology characteristics - ✔✔
Which grows better on MSA (Mannitol Salt Agar) staphylococcus or
enterococcus? Why? - ✔✔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? - ✔✔The Bile salts in the BEA
are what make it selective
2
100% Pass Guarantee Katelyn Whitman, All Rights
