Microbiology Lab Exam Study Guide
2023/ 31 Questions and solved
Answers/ Verified.
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 - -
-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
2023/ 31 Questions and solved
Answers/ Verified.
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 - -
-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
