M118L Quiz 1 Questions With Complete Solutions
A student writes the following two sentences in an exam:
Mannitol Salt Agar (MSA) medium contains a high
concentration (about 7.5%-10%) of salt (NaCl), allowing for
Gram-positive bacteria to grow and inhibiting others. This is the
reason why MSA agar is considered a differential medium.
Given that the first sentence is correct, is the student correct in
writing the second sentence? In other words, is the student
correct in his/her interpretation? Correct Answers The student
is incorrect in their interpretation. Because the growth of other
bacteria is inhibited, the medium should be considered
SELECTIVE.
After gram staining, what color will the gram negative cells be?
The gram positive? Correct Answers G-: pink
G+: purple
For the anal culture isolation, on EMB plates, you have been
asked to pick colonies which are gram negative, and those which
ferment lactose producing acid. How can you tell which colonies
to pick? Correct Answers EMB is a selective, differential
media, meaning that it will inhibit the growth of gram-positive
organisms. All colonies grown on the EMB plate will be gram-
negative. Additionally, gram-negative bacteria that ferments
glucose will produce a purple/green metallic sheen.
, Give an example of a differential medium. Correct Answers
MacConkey sugar → contains the pH indicator neutral red;
organisms that ferment lactose, a sugar in the media, produce an
acid end product... this lowers the pH, causing the lactose
fermenting colonies to be red, and the cells that do not ferment
lactose to be colorless
Give an example of a selective medium. Correct Answers
MacConkey sugar → used to grow gram negative bacteria;
possesses bile salts, which preferentially inhibit the growth of
gram positive organisms by specifically destroying the cell walls
of these organisms
Give an example of an enriched medium. Correct Answers
chocolate agar → contains red blood cells that have been lysed
through heating; permits the growth of organisms that require
specific growth factors found inside red blood cells
How do you know if cells have catalase? Correct Answers To
detect the presence of catalase, you subject your bacteria to
hydrogen peroxide. Those that possess catalase will generate
bubbles, demonstrating the break down of hydrogen peroxide
and the formation of oxygen gas.
If my phenol red control is cloudy, what does this mean? What
should I do next? (Proceed normally or something else?)
Correct Answers Proceed normally; only be concerned if your
phenol red control changes color.
Name the media used for minimal media test? Why is it called a
defined media? Correct Answers The media used for the media
A student writes the following two sentences in an exam:
Mannitol Salt Agar (MSA) medium contains a high
concentration (about 7.5%-10%) of salt (NaCl), allowing for
Gram-positive bacteria to grow and inhibiting others. This is the
reason why MSA agar is considered a differential medium.
Given that the first sentence is correct, is the student correct in
writing the second sentence? In other words, is the student
correct in his/her interpretation? Correct Answers The student
is incorrect in their interpretation. Because the growth of other
bacteria is inhibited, the medium should be considered
SELECTIVE.
After gram staining, what color will the gram negative cells be?
The gram positive? Correct Answers G-: pink
G+: purple
For the anal culture isolation, on EMB plates, you have been
asked to pick colonies which are gram negative, and those which
ferment lactose producing acid. How can you tell which colonies
to pick? Correct Answers EMB is a selective, differential
media, meaning that it will inhibit the growth of gram-positive
organisms. All colonies grown on the EMB plate will be gram-
negative. Additionally, gram-negative bacteria that ferments
glucose will produce a purple/green metallic sheen.
, Give an example of a differential medium. Correct Answers
MacConkey sugar → contains the pH indicator neutral red;
organisms that ferment lactose, a sugar in the media, produce an
acid end product... this lowers the pH, causing the lactose
fermenting colonies to be red, and the cells that do not ferment
lactose to be colorless
Give an example of a selective medium. Correct Answers
MacConkey sugar → used to grow gram negative bacteria;
possesses bile salts, which preferentially inhibit the growth of
gram positive organisms by specifically destroying the cell walls
of these organisms
Give an example of an enriched medium. Correct Answers
chocolate agar → contains red blood cells that have been lysed
through heating; permits the growth of organisms that require
specific growth factors found inside red blood cells
How do you know if cells have catalase? Correct Answers To
detect the presence of catalase, you subject your bacteria to
hydrogen peroxide. Those that possess catalase will generate
bubbles, demonstrating the break down of hydrogen peroxide
and the formation of oxygen gas.
If my phenol red control is cloudy, what does this mean? What
should I do next? (Proceed normally or something else?)
Correct Answers Proceed normally; only be concerned if your
phenol red control changes color.
Name the media used for minimal media test? Why is it called a
defined media? Correct Answers The media used for the media
