Post lab questions
Explain why a simple stain colorizes the cells while a negative stain does not.
Sometimes you can stain with both a simple stain and a negative stain at the same time. If
stained a smear of Bacillus subtilis with both safranin (pink) AND nigrosin (dark purple/black
negative stain) what would expect to see on the slide? Draw me a picture. Be sure to address the
shape of the bacteria in your drawing.
How about a smear of Streptococcus pyogenes with crystal violet (purple)? Draw a picture. Be
sure to address the shape and arrangement of the bacteria in your drawing (we didn’t look at this
one, you can tell the shape and arrangement from the name).
How about a smear of Staphylococcus aureus with Congo red? Draw a picture. Be sure to address
the shape and arrangement of the bacteria in your drawing.
Refer back to the picture you drew in your data section of the cells you see in the preparation of
your gumline slide.
There are some eukaryotic cells. How do you know that these are eukaryotic cells and not
prokaryotic? List at least two reasons.
Explain why eukaryotic cells are not considered gram-positive or gram-negative cells.
While they might not be gram positive or negative, do the eukaryotic cells stain like a gram-
positive cell or a gram-negative cell? Does your result on your gram stain make sense based
on what you know about the gram stain procedure and the difference between eukaryotic and
prokaryotic cells?
Both acid fast and non-acid fast cells are stained by carbolfuchsin. How is it, then, that we can
use it as a differential stain?
Explain how an acid-fast cell wall is different than a gram negative or gram-positive cell wall.
Remember that a capsule is a form of glycocalyx. What are the two reasons that a capsule is
considered a virulence factor?
Explain why bacteria that can make endospores have an advantage over those that can’t.
What differences did you see between the number AND location of endospores in the older
culture of Bacillus compared to a new culture? Why was there a difference?
When completed correctly, does a negative result for the endospore stain prove conclusively that
the organism doesn’t make endospores? Why or why not? When completed correctly, does a
positive result prove conclusively that the organism makes endospores? Why or why not?
