1. Name three underlying structural differences between Gram-positive and Gram-negative cell walls that explain why these different cell types can be distinguished from each other using the Gram stain.
2. When B. megaterium cells are Gram stained, the cells may have white (colorless) ovals inside the cells. What are these ovals and why are they colorless? (Hint: Have a look at Lab 8.)
3. You do a Gram stain with a pure culture of B. megaterium. When you look at the cells, half of the cells are purple and half of the cells are red. Assuming that your pure culture really was pure, explain why you see this result.
4. You do a Gram stain with a pure culture of E. coli. When you look at the cells, half of the cells are purple and half of the cells are red. Assuming that your pure culture really was pure, explain why you see this result.
5. You are working in a clinical laboratory engaged in disease diagnosis, and you receive a bacteria-rich sample from an infected patient. Why would you start by doing a Gram stain instead of a simple stain (Lab 5)? After all, the Gram stain takes more steps than a simple stain. In this context, why is Gram staining preferable to simple staining?
6. If you performed the Gram stain, but forgot to use the ethanol, what color would Gram-positive bacteria be at the end of the procedure? Assuming that you repeat the same mistake, what color would Gram-negative bacteria be at the end of the procedure? In both cases, be sure to explain why you drew the conclusions that you did.
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