Question 1

Which of the following best describes the main difference between chemically defined and complex media used for microbial growth?

Accepted Answer

Chemically defined media have an exact known chemical composition, while complex media contain extracts and digests of natural products with variable composition.

Question 2

A microbiologist wants to culture an obligate anaerobe. Which of the following methods would be most appropriate to ensure the survival and growth of this organism?

Accepted Answer

Use a reducing medium containing thioglycolate and store in a tightly capped tube.

Question 3

Which of the following best explains why food preservation techniques such as salting or sugaring are effective?

Accepted Answer

They create a hypertonic environment, causing plasmolysis in most bacteria.

Question 4

A bacterium is placed in a solution with 10% NaCl. If it is not salt-tolerant, what will most likely happen to the cell?

Accepted Answer

The cell will undergo plasmolysis as water leaves the cell.

Question 5

Which of the following is a primary reason why bacteria in biofilms are more resistant to antibiotics than planktonic bacteria?

Accepted Answer

The extracellular matrix of biofilms shelters bacteria from harmful substances.

Question 6

Which of the following best describes the function of superoxide dismutase in aerobic bacteria?

Accepted Answer

It converts superoxide radicals into hydrogen peroxide and oxygen: $$2 O_2$^- + 2 H^+ \rightarrow H_2O_2 + O_2$

Question 7

A culture medium contains 5 g peptone, 3 g beef extract, 8 g NaCl, and 15 g agar per liter. What type of medium is this?

Accepted Answer

Complex medium

Question 8

Which group of bacteria would you expect to find growing in a refrigerator at 4°C?

Accepted Answer

Psychrophiles

Question 9

Which of the following best explains why obligate anaerobes are killed by oxygen?

Accepted Answer

They lack enzymes such as superoxide dismutase and catalase to detoxify reactive oxygen species.

Question 10

A microbiologist wants to isolate a pure culture from a mixed sample. Which method should they use?

Accepted Answer

Streak-plate method

Question 11

Which of the following best describes the stationary phase of the bacterial growth curve?

Accepted Answer

The rate of cell division equals the rate of cell death.

Question 12

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

Accepted Answer

Plate count using serial dilutions and CFUs

Question 13

Which of the following best explains why facultative anaerobes grow better in the presence of oxygen?

Accepted Answer

They use oxygen as a final electron acceptor, producing more ATP via aerobic respiration.

Question 14

Which of the following is a use for phosphorus in microbial cells?

Accepted Answer

Component of phospholipids in cell membranes

Question 15

A bacterial culture is preserved by rapid cooling to –80°C in a suspension liquid. This method is known as:

Accepted Answer

Deep-freezing

Question 16

Which of the following best describes the role of biofilms in healthcare-associated infections?

Accepted Answer

Biofilms are responsible for most nosocomial infections due to their resistance to disinfectants and antibiotics.

Question 17

Which of the following is a correct description of the bacterial growth curve phase where cells are most susceptible to antibiotics targeting cell wall synthesis?

Accepted Answer

Log (exponential) phase

Question 18

Which of the following best describes the function of selective media?

Accepted Answer

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

Question 19

Which of the following statements about generation time is correct?

Accepted Answer

Generation time is the time required for a cell to divide and its population to double.

Question 20

A microbiologist is using MacConkey agar to isolate bacteria from a water sample. Which of the following is true about this medium?

Accepted Answer

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

Microbial Growth: Physical and Chemical Requirements, Culture Methods, and Measurement

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