Microbiology Lab Report #5 – Spring 2015
Name:
OBSERVATIONS AND INTERPRETATIONS
Organism or Source
Gram Stain
Color (+/-)
Differential Stains
Acid Fast Stain
Color (+/-)
Spore Stain
(+/-)
S. epidermidis
E. coli
N/A
M. smegmatis
N/A
B. megaterium
N/A
N/A
N/A
N/A
Questions:
1. Predict the effect of the following ‘mistakes’ made when performing a Gram
stain. Your answer needs to address the predicted effect on both Grampositive and Gram-negative cells. Consider each mistake independently.
a. Failure to add the iodine
b. Failure to apply the decolorizer
c. Failure to apply the safranin
d. Reversal of crystal violet and safranin stains
2. One of your lab partners has followed the recommended procedure of
running Gram-positive and Gram-negative control organisms on her Gram
stain of an unknown species. Her choices of controls were E. coli and Bacillus
subtilis. She tries several times and each time concludes she is decolorizing
too long because both controls have pink cells (E. coli has more than the B.
subtilis). What might you suggest she try and why?
3. Another lab partner has grown up an overnight culture of B. megaterium for
spore staining analysis. When he stains the culture, he does not see any
stains. Assuming that he has not contaminated his culture, and that he has
performed the spore stain perfectly, why did his spore stain not work? What
advice do you have for him?