Q: A bacterial culture starts with 200 cells and has a generation time of 30 minutes. If the culture grows exponentially for 3 hours, how many cells will be present at the end? Use the formula $$N = N_0$ \times 2^n$$, where $$N_0 is $$the initial number of cells and $$n is $$the number of generations.

$$N = 200 \times 2^6$ = 12,800$$ cells

Question 1

A bacterial culture starts with 200 cells and has a generation time of 30 minutes. If the culture grows exponentially for 3 hours, how many cells will be present at the end? Use the formula $$N = N_0$ 	imes 2^n$$, where $$N_0 is $$the initial number of cells and $$n is $$the number of generations.

Accepted Answer

$$N = 200 	imes 2^6$ = 12,800$$ cells

Question 2

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

Accepted Answer

Catalase converts hydrogen peroxide ($$H_2O_2) $$into water and oxygen, protecting the cell from oxidative damage.

Question 3

A microbiologist wants to encourage the growth of halophilic bacteria in a culture medium. Which additive should be increased in the medium?

Accepted Answer

Sodium chloride (NaCl)

Question 4

Which phase of bacterial growth in a broth culture is characterized by the fastest rate of cell division?

Accepted Answer

Exponential (log) phase

Question 5

Why is the stationary phase of bacterial growth called 'stationary'?

Accepted Answer

The rate of cell division equals the rate of cell death, so the total number of viable cells remains constant.

Question 6

Which of the following best explains why silver nitrate drops were historically used in the eyes of newborns?

Accepted Answer

To prevent infection by Neisseria gonorrhoeae, which can cause blindness.

Question 7

A microbiologist is using a spectrophotometer to measure the turbidity of a bacterial culture. What is being measured, and what is the limitation of this method?

Accepted Answer

Turbidity measures optical density, which correlates with cell mass, but it cannot distinguish between live and dead cells.

Question 8

Which of the following statements about pasteurization methods is correct?

Accepted Answer

Ultra pasteurization uses $$140^\circ C$$ for less than 1 second.

Question 9

Which of the following best distinguishes between sterilization and disinfection?

Accepted Answer

Sterilization destroys all forms of microbial life, including spores; disinfection eliminates most pathogens but not necessarily spores.

Question 10

A hospital is considering using ethylene oxide for sterilizing medical equipment. What is a key advantage of this method?

Accepted Answer

Ethylene oxide is a gas that can sterilize heat-sensitive materials without damaging them.

Question 11

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

Accepted Answer

It converts superoxide radicals ($$O_2$$^{\bullet-}$$) into hydrogen peroxide (H_2O_2$) and oxygen.

Question 12

A microbiologist performs a serial dilution and plate count to estimate the number of bacteria in a sample. Why is the result expressed as 'cfu/ml'?

Accepted Answer

Because each colony-forming unit (cfu) may arise from a single cell or a group of cells, and the value represents viable cells per milliliter.

Question 13

Which of the following is a reason why triclosan is no longer allowed in soaps?

Accepted Answer

Concerns about environmental persistence and the development of bacterial resistance.

Question 14

Which of the following best describes the effect of freezing at $$-20^\circ C on $$bacterial cultures?

Accepted Answer

It is bacteriostatic, inhibiting growth but not killing the cells.

Question 15

A culture medium is supplemented with high concentrations of sugar. Which type of microorganism is most likely to thrive in this environment?

Accepted Answer

Osmophiles

Question 16

Which of the following statements about the use of alcohols as disinfectants is correct?

Accepted Answer

Alcohols are most effective at concentrations between 60% and 95%.

Question 17

A patient is infected with an obligate anaerobe. In a deep tube of agar with oxygen only in the top 1-2 cm, where would you expect this organism to grow?

Accepted Answer

At the bottom of the tube, away from oxygen.

Question 18

Which of the following is a correct example of a mutualistic symbiosis?

Accepted Answer

Bacteria in the human gut that produce vitamins in exchange for nutrients.

Question 19

Which of the following physical methods of microbial control is most effective for sterilizing heat-sensitive medical equipment?

Accepted Answer

Filtration using a membrane filter

Question 20

Which of the following best describes the action of quaternary ammonium compounds?

Accepted Answer

They act as both detergents and antimicrobial agents, disrupting cell membranes.

BIO 255 Chapters 6 & 7 Microbial Growth and Control Study Guide – Step-by-Step Guidance

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