Q: A bacterial culture starts with 100 cells. If the population doubles every 30 minutes, how many cells will there be after 2 hours? 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.

$$N = 100 \times 2^4$ = 1600$$ cells

Q: A patient sample is serially diluted and plated. After incubation, 50 colonies are counted on a plate inoculated with 0.1 mL of a $$10^{-4}$$ dilution. What is the original concentration of bacteria in the sample (in cfu/mL)? Use the formula: $$\text{cfu/mL} = \frac{\text{number of colonies}}{\text{volume plated (mL)} \times \text{dilution factor}}.$$

$$\text{cfu/mL} = \frac{50}{0.1 \times 10$$^{-4}$$} = 5 \times 10^6$

Question 1

A bacterial culture starts with 100 cells. If the population doubles every 30 minutes, how many cells will there be after 2 hours? 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

$$N = 100 	imes 2^4$ = 1600$$ cells

Question 2

Which phase of the bacterial growth curve is characterized by a balance between cell division and cell death, resulting in no net increase in cell number?

Accepted Answer

Stationary phase

Question 3

A microbiologist is culturing bacteria on a solid medium. She notices that the oldest cells are located in the center of the colony. What is the most likely explanation for this observation?

Accepted Answer

Cells in the center were the first to divide and have been present the longest.

Question 4

Which of the following best describes a facultative anaerobe?

Accepted Answer

It can grow with or without oxygen, but grows better with oxygen.

Question 5

A bacterial species is found to lack catalase and superoxide dismutase. Which oxygen requirement category does it most likely belong to?

Accepted Answer

Obligate anaerobe

Question 6

Which of the following is a function of siderophores in bacterial cells?

Accepted Answer

They bind and transport iron into the cell.

Question 7

A bacterium grows best at 37°C but can survive between 10°C and 45°C. To which temperature classification does it belong?

Accepted Answer

Mesophile

Question 8

Which of the following best explains why acidophiles can survive in environments with low pH?

Accepted Answer

They possess cellular mechanisms to pump out excess protons and maintain internal pH.

Question 9

A researcher wants to determine the number of viable bacteria in a sample. Which method should she use and why?

Accepted Answer

Plate count method, because it only counts live cells capable of forming colonies.

Question 10

Which of the following chemical elements is NOT typically required in large amounts for bacterial growth?

Accepted Answer

Iron

Question 11

A culture of bacteria is exposed to singlet oxygen ($$^1O_2$$), hydrogen peroxide ($$H_2O_2$$), and superoxide ($$O_2$$^-$$). Which enzymes would protect the bacteria from these toxic forms of oxygen?

Accepted Answer

Catalase, superoxide dismutase, and peroxidase

Question 12

A bacterial culture is measured using spectrophotometry. The optical density (OD) reading increases over time. What does this indicate about the culture?

Accepted Answer

The cell population is increasing, but both live and dead cells contribute to turbidity.

Question 13

A patient sample is serially diluted and plated. After incubation, 50 colonies are counted on a plate inoculated with 0.1 mL of a $$10^{-4}$$ dilution. 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

$$	ext{cfu/mL} = \frac{50}{0.1 	imes 10$$^{-4}$$} = 5 	imes 10^6$

Question 14

Which of the following best describes the lag phase of bacterial growth?

Accepted Answer

Cells are metabolically active but not dividing rapidly as they adjust to new conditions.

Question 15

A bacterium isolated from deep-sea hydrothermal vents grows optimally at 90°C. Which term best describes this organism?

Accepted Answer

Hyperthermophile

Question 16

Which of the following statements about direct microscopic counts is correct?

Accepted Answer

They count both live and dead cells.

Question 17

A laboratory technician is culturing a bacterium that requires organic compounds as a carbon source and cannot synthesize certain vitamins. What are these vitamins called in the context of bacterial nutrition?

Accepted Answer

Growth factors

Question 18

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

Accepted Answer

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

Question 19

A foodborne pathogen is able to grow at refrigerator temperatures (4°C) but grows faster at room temperature (25°C). Which term best describes this organism?

Accepted Answer

Psychrotroph

Question 20

A scientist is studying a microbe that grows best in environments with low oxygen concentrations, such as the human stomach. Which term best describes this organism?

Accepted Answer

Microaerophile

Microbial Growth: Characteristics, Requirements, and Measurement

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