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Microbiology Lab Exam Study Guide
Questions And Answers 100% Pass.
What color do Gram-Positive bacteria stain? - Answer✔Purple
What color do Gram-Negative bacteria stain? - Answer✔Pink
What are the 4 steps of a gram reaction? - Answer✔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? -
Answer✔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) - Answer✔
Look over colony morphology characteristics - Answer✔
Which grows better on MSA (Mannitol Salt Agar) staphylococcus or enterococcus? Why? -
Answer✔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? - Answer✔The Bile salts in the BEA are what make
it selective
What is in BEA agar that makes it Differential - Answer✔The Esculin Azide in the agar is what
makes it differential
Microbiology Lab Exam Study Guide
Questions And Answers 100% Pass.
What color do Gram-Positive bacteria stain? - Answer✔Purple
What color do Gram-Negative bacteria stain? - Answer✔Pink
What are the 4 steps of a gram reaction? - Answer✔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? -
Answer✔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) - Answer✔
Look over colony morphology characteristics - Answer✔
Which grows better on MSA (Mannitol Salt Agar) staphylococcus or enterococcus? Why? -
Answer✔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? - Answer✔The Bile salts in the BEA are what make
it selective
What is in BEA agar that makes it Differential - Answer✔The Esculin Azide in the agar is what
makes it differential
