Question 1

Which of the following best describes the purpose of selective media in microbiology?

Accepted Answer

To suppress unwanted microbes and encourage the growth of desired microbes.

Question 2

A microbiologist wants to isolate Staphylococcus species from a mixed sample. Which medium should they use and why?

Accepted Answer

Mannitol Salt Agar, because its high salt concentration inhibits most bacteria except Staphylococcus.

Question 3

Which of the following statements about MacConkey agar is correct?

Accepted Answer

It is selective for Gram-negative bacteria and differential for lactose fermentation.

Question 4

A bacterial colony on Mannitol Salt Agar turns the medium yellow. What does this indicate?

Accepted Answer

The bacteria are Staphylococcus species that ferment mannitol, producing acid.

Question 5

Which of the following equations represents the reaction catalyzed by superoxide dismutase?

Accepted Answer

$$O_2^- + O_2^- + 2H^+ \rightarrow H_2O_2 + O_2$$

Question 6

Which type of hemolysis on blood agar is characterized by a clear zone around the colonies due to complete lysis of red blood cells?

Accepted Answer

Beta hemolysis

Question 7

A microbiologist observes colorless colonies on Eosin Methylene Blue (EMB) agar. What does this indicate about the bacteria?

Accepted Answer

They are Gram-negative and do not ferment lactose.

Question 8

Which of the following best describes an obligate anaerobe?

Accepted Answer

Cannot use oxygen and is harmed by its presence.

Question 9

Which medium would be most appropriate for cultivating fastidious organisms that require additional nutrients?

Accepted Answer

Enriched media

Question 10

A tube of thioglycollate broth shows bacterial growth only at the bottom of the tube. What is the oxygen requirement of this organism?

Accepted Answer

Obligate anaerobe

Question 11

Which of the following best explains why phenylethyl alcohol agar (PEA) is selective for Gram-positive bacteria?

Accepted Answer

PEA contains phenylethyl alcohol, which inhibits the growth of Gram-negative bacteria.

Question 12

Which of the following is a correct pairing of media type and its primary purpose, according to Table 6.5?

Accepted Answer

Complex media: Growth of most chemoheterotrophic organisms

Question 13

A microbiologist is testing an unknown bacterium on MacConkey agar. After incubation, the colonies appear red. What can be concluded?

Accepted Answer

The bacterium is Gram-negative and ferments lactose.

Question 14

Which of the following best describes the function of catalase in bacterial cells?

Accepted Answer

Catalase converts hydrogen peroxide ($$H_2O_2) $$into water and oxygen.

Question 15

Which of the following best describes a facultative anaerobe?

Accepted Answer

Can grow with or without oxygen, but grows better with oxygen.

Question 16

A microbiologist observes a green metallic sheen on EMB agar. Which organism is most likely present?

Accepted Answer

Escherichia coli

Question 17

Which of the following best describes the role of enrichment media?

Accepted Answer

Increases the number of desired microbes to detectable levels.

Question 18

Which of the following is a toxic oxygen product that is neutralized by superoxide dismutase?

Accepted Answer

Superoxide radical ($$O_2^-)$$

Question 19

A microbiologist observes incomplete lysis of red blood cells around colonies on blood agar. What type of hemolysis is this?

Accepted Answer

Alpha hemolysis

Question 20

Which of the following best describes a microaerophile?

Accepted Answer

Requires oxygen at a concentration lower than air.

Microbial Culture Media and Oxygen Requirements

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