Question 1

Which of the following best describes a pure culture in microbiology?

Accepted Answer

A culture containing only one kind of microorganism, all derived from the same original cell.

Question 2

What is the main purpose of the streak plate method?

Accepted Answer

To obtain a pure culture of a specific bacterium present within a mixture.

Question 3

During which phase of the bacterial growth curve does the population size remain relatively constant because the rate of cell division equals the rate of cell death?

Accepted Answer

Stationary phase

Question 4

If a single bacterium divides every 20 minutes, how many cells will there be after 2 hours?

Accepted Answer

$$2^6 = 64$

Question 5

Which step of binary fission is directly affected by antibiotics that inhibit DNA synthesis?

Accepted Answer

First step: DNA replication

Question 6

Why do microbiologists often use a logarithmic scale to plot bacterial growth?

Accepted Answer

Because exponential growth becomes a straight line, making it easier to compare growth rates.

Question 7

Which of the following is a disadvantage of the plate count method for measuring bacterial populations?

Accepted Answer

The time required is not ideal for all applications.

Question 8

Which direct method would be most appropriate for estimating a very small number of bacteria in a large water sample?

Accepted Answer

Filtration followed by culturing on a membrane.

Question 9

Which indirect method is useful for estimating the amount of vitamins in a sample by measuring a metabolic product?

Accepted Answer

Metabolic activity

Question 10

A microbiologist wants to preserve a bacterial culture for several years. Which method should they use?

Accepted Answer

Deep-freezing or freeze-drying (lyophilization)

Question 11

Which of the following pairs is mismatched regarding the bacterial growth curve?

Accepted Answer

Stationary phase — cell death rate is greater than cell birth rate

Question 12

Why might 'counting every cell' be difficult when bacterial populations become enormous?

Accepted Answer

Because the number of cells increases exponentially, making manual counting impractical.

Question 13

Which method would NOT be suitable for measuring the growth of filamentous bacteria or molds?

Accepted Answer

Plate count

Question 14

In the streak plate method, why are isolated colonies more likely to appear near the final streaks?

Accepted Answer

Because cell density decreases with each streak, allowing individual cells to grow into separate colonies.

Question 15

Which of the following best explains why exponential growth of bacteria eventually stops in a closed system?

Accepted Answer

Nutrients become limiting, wastes accumulate, and space becomes scarce.

Question 16

A sample is diluted 1:1000 and 0.1 mL is plated. If 50 colonies are counted, what is the original concentration of bacteria per mL?

Accepted Answer

$$50 	imes 10 	imes 1000 = 500,000$$ CFU/mL

Question 17

Which direct method can count both live and dead cells in a sample?

Accepted Answer

Direct microscopic count

Question 18

Which of the following is a key limitation of the pour plate method compared to the spread plate method?

Accepted Answer

Heat-sensitive cells may be damaged by melted agar in the pour plate method.

Question 19

Which of the following is NOT an indirect method for measuring microbial growth?

Accepted Answer

Direct microscopic count

Question 20

Which phase of the bacterial growth curve is characterized by intense metabolic activity but little or no increase in cell number?

Accepted Answer

Lag phase

Pure Cultures and Bacterial Growth: Isolation, Preservation, and Measurement

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