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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
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 - ✔✔
Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE
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
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 - ✔✔
Created by Katelyn Whitman © 2025, All Rights Reserved. 100% PASS GUARANTEE
