Q: A culture of *E. coli* is started with 100 cells. After 2 hours, there are 1600 cells. What is the generation time? Use the formula $$\text{Generation Time} = \frac{\text{Growth Time}}{\text{Number of Generations}}$$ and show your calculation.

30 minutes; $$\text{Number of generations} = \log_2$(1600/100) = 4$$, $$\text{Generation Time} = 120/4 = 30$$ minutes

Question 1

Which tube in the first image best represents the growth pattern of a facultative anaerobe, and why?

Accepted Answer

Tube C, because facultative anaerobes grow throughout the tube but more densely at the top where oxygen is present.

Question 2

During which phase of the bacterial growth curve (second image) are bacteria most susceptible to antibiotics that target cell wall synthesis, and why?

Accepted Answer

Log phase, because cells are actively dividing and synthesizing new cell walls.

Question 3

Which step in the biofilm formation process (third image) is most critical for the development of antibiotic resistance within the biofilm community?

Accepted Answer

Step 4: Biofilm maturation; increased cell density and intercellular signaling.

Question 4

A bacterium isolated from a hot spring grows optimally at 70°C and can survive at temperatures up to 120°C. Which term best describes this organism?

Accepted Answer

Extreme thermophile

Question 5

Which of the following best explains why biofilms are more resistant to antimicrobial agents than planktonic cells?

Accepted Answer

Biofilms contain a protective extracellular matrix that limits antimicrobial penetration.

Question 6

A culture of *E. coli* is started with 100 cells. After 2 hours, there are 1600 cells. What is the generation time? Use the formula $$	ext{Generation Time} = \frac{	ext{Growth Time}}{	ext{Number of Generations}}$$ and show your calculation.

Accepted Answer

30 minutes; $$	ext{Number of generations} = \log_2$(1600/100) = 4$$, $$	ext{Generation Time} = 120/4 = 30$$ minutes

Question 7

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

Accepted Answer

It converts superoxide ions to hydrogen peroxide.

Question 8

Which type of medium would be most appropriate for isolating a fastidious organism that requires multiple growth factors?

Accepted Answer

Complex medium

Question 9

A patient sample is suspected to contain an obligate anaerobe. Which of the following methods would best ensure the survival of this organism during transport to the laboratory?

Accepted Answer

Transport in a sealed anaerobic jar with oxygen-reacting chemicals.

Question 10

Which of the following statements about halophiles is TRUE?

Accepted Answer

Halophiles require high salt concentrations for growth.

Question 11

Which physical method is most effective for sterilizing heat-sensitive nutrient broth without altering its composition?

Accepted Answer

Membrane filtration with a 0.22 μm filter

Question 12

Which of the following best describes the action of ethylene oxide as a sterilizing agent?

Accepted Answer

It alkylates proteins and DNA, preventing normal cellular functions.

Question 13

A laboratory is using a spectrophotometer to measure the turbidity of a bacterial culture. What is being indirectly measured?

Accepted Answer

The total cell population, both living and dead

Question 14

Which of the following best explains why *Staphylococcus aureus* can grow on mannitol salt agar (MSA)?

Accepted Answer

It is halophilic and can tolerate high salt concentrations.

Question 15

Which of the following is the correct order of the bacterial growth curve phases as shown in the second image?

Accepted Answer

Lag phase → Log phase → Stationary phase → Death phase

Question 16

Which of the following best describes the function of a chemostat in microbial culture?

Accepted Answer

It maintains cellular growth at a constant rate by continuously adding fresh medium and removing waste.

Question 17

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

Accepted Answer

Catalase converts hydrogen peroxide to water and oxygen.

Question 18

A sample is serially diluted and plated using the spread plate method. After incubation, 50 colonies are counted on a plate that received 0.1 mL of a 1:1000 dilution. What is the original concentration of bacteria in CFU/mL? Use the formula $$	ext{CFU/mL} = \frac{	ext{Number of colonies}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}}.$$

Accepted Answer

$$\frac{50}{0.1 	imes 0.001} = 500,000$$ CFU/mL

Question 19

Which of the following best describes the difference between selective and differential media?

Accepted Answer

Selective media suppress the growth of certain microbes; differential media visually distinguish microbes based on metabolic properties.

Fundamentals of Microbial Growth and Control

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