Question 1

Which of the following best describes the role of nitrogen-fixing bacteria in the nitrogen cycle?

Accepted Answer

They convert atmospheric nitrogen (N₂) into ammonia (NH₃), making nitrogen available to living organisms.

Question 2

A bacterium that uses carbon dioxide (CO₂) as its sole carbon source and obtains energy from inorganic chemical reactions is classified as which of the following?

Accepted Answer

Chemoautotroph

Question 3

During the process of mineralization (ammonification), what is the primary role of microbes in the soil?

Accepted Answer

They break down organic nitrogen compounds and release ammonium (NH₄⁺).

Question 4

Which of the following transport mechanisms requires direct use of ATP to move molecules against their concentration gradient?

Accepted Answer

Primary active transport

Question 5

A microbiologist wants to isolate a pure culture from a mixed sample. Which method is most appropriate for achieving this goal?

Accepted Answer

Streak plate method on agar

Question 6

Why do many pathogenic bacteria produce siderophores during infection?

Accepted Answer

To scavenge iron from the host, overcoming nutritional immunity

Question 7

Which phase of the microbial growth curve is characterized by metabolic activity without an increase in cell number?

Accepted Answer

Lag phase

Question 8

A bacterial culture is incubated at 37°C in a sealed jar with no oxygen. Which type of bacteria is most likely to thrive under these conditions?

Accepted Answer

Anaerobic bacteria

Question 9

A plate contains 0.1 mL of a 10⁻⁴ dilution of a bacterial culture and yields 120 colonies after incubation. What is the original concentration of bacteria in the sample (in CFU/mL)?

Use the formula: $$	ext{CFU/mL} = \frac{	ext{Number of colonies}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}}$$

Accepted Answer

$$1.2 	imes 10^6$ CFU/mL

Question 10

Which of the following best explains why facilitated diffusion is considered a type of passive transport?

Accepted Answer

It uses membrane proteins to move molecules down their concentration gradient without energy input.

Question 11

In the context of microbial nutrient uptake, what is the main advantage of group translocation?

Accepted Answer

It chemically modifies the nutrient during transport, preventing it from diffusing back out.

Question 12

During the stationary phase of bacterial growth, which of the following is most likely to occur?

Accepted Answer

Cell division rate equals cell death rate, and stress responses are activated.

Question 13

Which of the following statements about osmosis in microbial cells is correct?

Accepted Answer

Osmosis is the movement of water from low to high solute concentration across a semi-permeable membrane.

Question 14

A researcher observes that a bacterial culture stops growing after several days, even though the temperature and pH are optimal. Which of the following is the most likely explanation?

Accepted Answer

Nutrient depletion and waste accumulation have led to the stationary phase.

Question 15

Which of the following best describes the function of ammonium transporters in microbial cells?

Accepted Answer

They enable the uptake of ammonia (NH₃) and protons (H⁺), which recombine inside the cell to form ammonium (NH₄⁺).

Question 16

Which of the following is a key reason why only plates with 30–300 colonies are considered statistically reliable for CFU counts?

Accepted Answer

Plates with fewer than 30 colonies have high sampling error, and plates with more than 300 colonies have overcrowding and merging.

Question 17

Which of the following best explains why iron acquisition is a major virulence factor for many pathogenic microbes?

Accepted Answer

Iron is required for microbial metabolism, and the host limits its availability to prevent infection.

Question 18

A student is culturing an environmental microbe that grows best at 25°C and requires elevated CO₂ levels. Which of the following describes the optimal incubation conditions?

Accepted Answer

Incubate at 25°C in a CO₂-enriched atmosphere (5–10% CO₂).

Question 19

Which of the following scenarios best illustrates the process of passive transport in a microbial cell?

Accepted Answer

Oxygen diffuses directly through the cell membrane from high to low concentration.

Bacterial Culture, Growth, and Nutrient Acquisition: Microbiology Study Notes

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