Microbiology lab homework #3
Spring 2014
Prof. Santos
1-The counter stain for the acid fast stain is
a) methylene blue
b) Iodine
c) Safranin
d) crystal violet.
2- True or False
Bacillus megaterium is a gram + rod.
3- Gram + bacteria retain the primary stain and appear purple mainly because
a) thick layer of peptidoglycan
b) thin layer of peptidoglycan
c) the lipid A molecule of the LPS
d) teichoic acid molecules present in the cell wall
4- Which of the following is an acid fast microorganism?
a) E. coli
b) S. aureus
c) S. pneumonia
d) Mycobacterium smegmatis
5- Which stain is a differential stain?
a) gram stain
b) capsule stain
c) acid fast stain
d) both a and c
6-Safranin is the
a) primary stain for the Gram stain
b) primary stain for the endospore stain
c) counter stain for the Gram stain
d) counter stain for the acid fast stain
7- On the diagram below, draw an arrow to the part of the petri dish a student would most
likely use to obtain a colony of a single bacterium for identification purpose?
8- List 3 factors that must be considered when doing a Gram stain that can affect the outcome of
the staining and give a student a false positive result.
a-
b-
c-
9- When making a smear, why must the preparation be thin?
10- In the acid fast stain, what is the primary stain and why is it applied over steam?