Q: If a bacterial culture starts with 1 cell and doubles every generation, how many cells will there be after 10 generations? 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.

$$N = 1 \times 2$$^{10}$$ = 1,024$$

Q: A student performs a serial dilution and plates 1 mL of a $$1/10,000$$ dilution. If 50 colonies are counted, what is the original concentration of bacteria in the sample? Use the formula: $$\text{Number of colonies} \times \text{reciprocal of dilution} = \text{number of bacteria/mL}.$$

$$50 \times 10,000 = 500,000$$ bacteria/mL

Q: If a culture has $$10^6$$ cells/mL at the start of stationary phase and the death phase reduces the population by 90% every hour, how many cells/mL remain after 2 hours?

$$10^6$$ \times 0.1 \times 0.1 = 10,000$$ cells/mL

Q: If you plate 0.1 mL of a $$1/100,000$$ dilution and count 20 colonies, what is the concentration of bacteria in the original sample? Use the formula: $$\text{Number of colonies} \times \text{reciprocal of dilution} \times \frac{1}{\text{volume plated in mL}}.$$

$$20 \times 100,000 \times 10 = 20,000,000$$ bacteria/mL

Q: A culture starts with 100 cells and doubles every 30 minutes. How many cells will there be after 3 hours? Use $$N = N_0$ \times 2^n$$, where $$n is $$the number of generations.

$$N = 100 \times 2^6$ = 6,400$$

Question 1

Which phase of the bacterial growth curve is characterized by a balance between cell division and cell death, resulting in a stable population size?

Accepted Answer

Stationary phase

Question 2

If a bacterial culture starts with 1 cell and doubles every generation, how many cells will there be after 10 generations? 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

$$N = 1 	imes 2$$^{10}$$ = 1,024$$

Question 3

During which phase of the bacterial growth curve does E. coli divide every 20 minutes under optimal conditions?

Accepted Answer

Log phase

Question 4

A student performs a serial dilution and plates 1 mL of a $$1/10,000$$ dilution. If 50 colonies are counted, what is the original concentration of bacteria in the sample? Use the formula: $$	ext{Number of colonies} 	imes 	ext{reciprocal of dilution} = 	ext{number of bacteria/mL}.$$

Accepted Answer

$$50 	imes 10,000 = 500,000$$ bacteria/mL

Question 5

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

Accepted Answer

Cells are adapting to a new environment and not dividing rapidly.

Question 6

If a culture has $$10^6$$ cells/mL at the start of stationary phase and the death phase reduces the population by 90% every hour, how many cells/mL remain after 2 hours?

Accepted Answer

$$10^6$$ 	imes 0.1 	imes 0.1 = 10,000$$ cells/mL

Question 7

Which of the following is a reason why the stationary phase occurs in a bacterial growth curve?

Accepted Answer

Cell division rate equals the cell death rate due to nutrient depletion and waste accumulation.

Question 8

A microbiologist wants to determine the number of viable bacteria in a sample. Which method should they use?

Accepted Answer

Serial dilution and plate count

Question 9

If you plate 0.1 mL of a $$1/100,000$$ dilution and count 20 colonies, what is the concentration of bacteria in the original sample? Use the formula: $$	ext{Number of colonies} 	imes 	ext{reciprocal of dilution} 	imes \frac{1}{	ext{volume plated in mL}}.$$

Accepted Answer

$$20 	imes 100,000 	imes 10 = 20,000,000$$ bacteria/mL

Question 10

Which of the following best explains why the log phase is the best time to study the effects of antibiotics on bacteria?

Accepted Answer

Bacteria are most metabolically active and dividing rapidly.

Question 11

A culture starts with 100 cells and doubles every 30 minutes. How many cells will there be after 3 hours? Use $$N = N_0$ 	imes 2^n$$, where $$n is $$the number of generations.

Accepted Answer

$$N = 100 	imes 2^6$ = 6,400$$

Question 12

Which of the following is NOT a characteristic of the death phase in the bacterial growth curve?

Accepted Answer

Population size increases rapidly.

Question 13

Why is it important to use serial dilutions when plating bacteria for colony counts?

Accepted Answer

To ensure that the colonies are countable and not too numerous to distinguish.

Question 14

If a bacterial culture has a generation time of 20 minutes, how many generations will occur in 4 hours?

Accepted Answer

$$\frac{4 	imes 60}{20} = 12$$ generations

Question 15

A student observes that after 24 hours, the number of colonies on a plate has decreased compared to the 12-hour plate. Which phase of the growth curve does this most likely represent?

Accepted Answer

Death phase

Question 16

Which of the following equations correctly represents the calculation for the number of bacteria in a sample using serial dilution and plate count?

Accepted Answer

$$	ext{Number of colonies} 	imes 	ext{reciprocal of dilution} 	imes \frac{1}{	ext{volume plated in mL}}$$

Dynamics of Microbial Growth: Growth Curves, Generation Time, and Quantification Methods

Study Guide - Practice Questions

Study Guide - Flashcards