Microbiology Lab Exam Study Guide 2025 Questions & Answers with 100% Correct 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 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