Question 1

Which of the following best explains why psychrotrophs are a concern in food safety, particularly in refrigerated foods?

Accepted Answer

They can grow at refrigeration temperatures and may cause food spoilage even when food is kept cold.

Question 2

A microbiologist is culturing a bacterium that requires many specific vitamins and amino acids to grow. Which type of medium should be used?

Accepted Answer

Chemically defined medium

Question 3

Which of the following equations correctly represents the total number of cells after $$n$$ generations, starting from a single bacterial cell dividing by binary fission?

Accepted Answer

$$2^n$$

Question 4

A bacterial culture is placed in a hypertonic solution containing 10% NaCl. What is the most likely outcome for the bacterial cells?

Accepted Answer

Plasmolysis will occur, inhibiting cell growth due to water leaving the cell.

Question 5

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

Accepted Answer

It neutralizes toxic superoxide radicals ($$O_2$$^-$$) produced during oxygen metabolism.

Question 6

A patient develops an infection on a catheter. Laboratory analysis reveals the presence of a biofilm. Which property of biofilms most likely contributed to the persistence of this infection?

Accepted Answer

Biofilms provide shelter from antibiotics and immune responses, making bacteria up to 1000x more resistant to microbicides.

Question 7

Which of the following best explains why phosphate salts are commonly used as buffers in microbial growth media?

Accepted Answer

They maintain a stable pH and provide essential phosphorus for microbial metabolism.

Question 8

A microbiologist wants to culture obligate anaerobes. Which method is most appropriate for creating an oxygen-free environment?

Accepted Answer

Using a reducing medium containing sodium thioglycolate and heating to drive off $$O_2$$

Question 9

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

Accepted Answer

The rate of cell division equals the rate of cell death, resulting in no net increase in population.

Question 10

A water sample is filtered and the filter is placed on a selective agar. 214 colonies are counted. What is the bacterial concentration in the original 100 mL sample?

Accepted Answer

214 bacteria per 100 mL

Question 11

Which of the following is a key difference between selective and differential media?

Accepted Answer

Selective media suppress unwanted microbes, while differential media distinguish between different microbes based on colony appearance.

Question 12

A bacterial culture is growing in a medium with a pH of 4.5. Which type of microorganism is most likely to thrive in this environment?

Accepted Answer

Acidophiles

Question 13

Which of the following best describes the role of trace elements in microbial growth?

Accepted Answer

They act as enzyme cofactors and are required in small amounts.

Question 14

A microbiologist is using the streak plate method to isolate a pure culture. What is the principle behind this technique?

Accepted Answer

It separates individual cells on the surface of an agar plate, allowing colonies to arise from single cells.

Question 15

Which of the following best explains why food preserved with high concentrations of salt or sugar resists spoilage?

Accepted Answer

High osmotic pressure causes plasmolysis in most microbes, inhibiting their growth.

Question 16

Which of the following best describes the growth pattern of obligate anaerobes in a tube of solid growth medium?

Accepted Answer

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

Question 17

A bacterial culture is in the log phase of growth. If the generation time is 30 minutes, how many cells will there be after 3 hours, starting from a single cell?

Accepted Answer

$$2^6 = 64$ cells

Question 18

Which of the following is an indirect method for estimating bacterial numbers in a culture?

Accepted Answer

Measuring turbidity with a spectrophotometer

Question 19

Which biosafety level (BSL) requires sealed, negative pressure laboratories and double HEPA filtration of exhaust air?

Accepted Answer

BSL-4

Question 20

Which of the following best explains why fastidious organisms require chemically defined media for growth?

Accepted Answer

They require many specific growth factors that must be provided in known quantities.

Microbial Growth: Requirements, Culture, and Measurement

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