Question 1

Which of the following best describes the function of reducing agents in media designed for anaerobic bacteria?

Accepted Answer

They remove oxygen from the medium, creating anaerobic conditions.

Question 2

A clinical sample is streaked onto MacConkey agar. After incubation, dark pink colonies are observed. What is the most accurate interpretation of this result?

Accepted Answer

The organism is a Gram-negative lactose fermenter.

Question 3

Which type of media would you use to isolate Staphylococcus aureus from a mixed culture, and what result would you expect if S. aureus is present?

Accepted Answer

Mannitol Salt Agar; yellow colonies due to mannitol fermentation

Question 4

A bacterium is described as a psychrophile. At which temperature would you expect it to grow optimally?

Accepted Answer

$$15^\circ \mathrm{C}$$

Question 5

Which of the following best describes the streak plate method?

Accepted Answer

It spreads a loopful of culture over solid media to obtain isolated colonies.

Question 6

A bacterial culture is in the stationary phase of growth. Which of the following best describes what is occurring?

Accepted Answer

The rate of cell death equals the rate of new cell formation.

Question 7

Which of the following media is both selective and differential, and what is its primary use?

Accepted Answer

Mannitol Salt Agar; for selecting Staphylococcus and differentiating mannitol fermenters

Question 8

What is the expected outcome when a bacterial cell is placed in a hypertonic solution?

Accepted Answer

The cell will undergo plasmolysis as water leaves the cell.

Question 9

Which of the following best defines 'generation time' in bacterial growth?

Accepted Answer

The time it takes for a bacterial population to double in number.

Question 10

A microbiologist wants to directly measure the number of viable bacteria in a water sample. Which method should they use?

Accepted Answer

Plate count method

Question 11

Which of the following best explains why MacConkey agar is considered both selective and differential?

Accepted Answer

It contains bile salts and crystal violet to inhibit Gram-positives and differentiates lactose fermenters by color change.

Question 12

A patient sample is cultured on Blood Agar. Colonies are surrounded by a clear zone. What does this indicate about the organism?

Accepted Answer

It is beta-hemolytic and completely lyses red blood cells.

Question 13

Which of the following best describes the growth pattern of an obligate anaerobe in sodium thioglycolate broth?

Accepted Answer

Growth only at the bottom of the tube where oxygen is absent.

Question 14

Which of the following media would be most appropriate for isolating pathogenic Neisseria species from a clinical specimen?

Accepted Answer

Thayer Martin Agar

Question 15

A bacterial culture is in the log (exponential) phase. Which of the following statements is true?

Accepted Answer

Cells are dividing at their maximum rate and population size is increasing exponentially.

Question 16

Which of the following best describes the purpose of using enriched media such as Chocolate agar?

Accepted Answer

To provide additional nutrients for the growth of fastidious organisms.

Question 17

A microbiologist observes that a bacterial culture has entered the death phase. Which equation best describes the change in cell number over time during this phase?

Accepted Answer

$$N_t = N_0$ e$$^{-kt}$$

Question 18

Which of the following is a direct method for measuring bacterial growth?

Accepted Answer

Plate count method

Question 19

A sample is cultured on Sabouraud Dextrose Agar and shows significant growth. What is the most likely identity of the organism?

Accepted Answer

Fungus (yeast or mold)

Question 20

Which of the following best explains why Mannitol Salt Agar is selective for Staphylococcus species?

Accepted Answer

It contains 7.5% NaCl, which inhibits most bacteria except Staphylococcus.

Microbial Growth and Culture Media: Guided Study and Practice

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