Microbiology Laboratory
Microbiology Interpretation and Questions
1. What would you observe if you decolorized your slide too much? How would your
... [Show More] cells
appear (make sure you understand what is going on in each step of the Gram stain; that is the
key to answering this question as well as questions 3 and 5)?
If the cells are over-decolorized, the primary stain (crystal violet) will be washed out of the
Gram-positive cells and all the cells will appear Gram-negative (pink) due to the counterstain
with safranin.
2. How would you describe the morphology and arrangement of the cells in your stained
preparation?
Staphylococcus aureus is a Gram-positive coccus that appears purple. Pseudomonas aeruginosa
is a Gram-negative
3. You are looking at the smear of the mixture. All of the cells, cocci and bacilli, appear deep
purple. What could have gone wrong?
If the slide was not sufficiently decolorized or if the decolorizing step was forgotten, where the
Gram-negative organisms will retain the primary stain (crystal violet) and the counter stain
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Lab Report 11
(safranin) will not be visible, causing all of the cells, both Gram-positive and Gram-negative, to
appear a deep purple.
4. Explain the relationship between the observed Gram reaction and bacterial cell wall
structure.
In a Gram-positive cell there is more peptidoglycan then in a Gram-negative cell. The gram
reaction dyes more of the gram positive because there is more peptidoglycan. Then going
through the process Gram-negative gets no color until safranin.
5. You mistakenly confuse the primary stain and counterstain. You initially stain the smear with
safranin, add iodine, and then decolorize and counterstain with crystal violet. How does you
fixed culture appear when viewed with oil immersion?
They would all look purple and it would be very hard to be able to distinguish between the two.
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