Q: A culture starts with 100 cells/ml and has a generation time of 1 hour. How many cells/ml will there be after 4 hours, assuming no lag phase and binary fission?

$$100 \times 2^4$ = 1,600$$ cells/ml

Q: A sample is diluted in a series of 1:10 dilutions, and 0.1 ml from the $$10^{-4}$$ dilution is plated, resulting in 65 colonies. What is the original concentration of bacteria in the sample?

$$65 \times 10^4$ \times 10 = 6.5 \times 10^6$ cells/ml

Question 1

Which of the following best describes a defined (synthetic) culture medium?

Accepted Answer

A medium in which the exact chemical composition is known and all components are chemically defined.

Question 2

A microbiologist wants to isolate and differentiate bacteria based on their ability to ferment lactose. Which medium should she use?

Accepted Answer

MacConkey agar

Question 3

Which of the following best explains why MacConkey agar is considered both selective and differential?

Accepted Answer

It inhibits Gram-positive bacteria and distinguishes lactose fermenters by color change.

Question 4

A blood agar plate shows clear zones around some colonies, greenish zones around others, and no change around the rest. What do these results indicate?

Accepted Answer

Beta-hemolysis, alpha-hemolysis, and gamma-hemolysis, respectively.

Question 5

Which phase of the bacterial growth curve is characterized by the highest susceptibility to antimicrobial drugs?

Accepted Answer

Log (exponential) phase

Question 6

A culture starts with 100 cells/ml and has a generation time of 1 hour. How many cells/ml will there be after 4 hours, assuming no lag phase and binary fission?

Accepted Answer

$$100 	imes 2^4$ = 1,600$$ cells/ml

Question 7

Which method would be most appropriate for counting bacteria in a water sample with very low cell density?

Accepted Answer

Membrane filtration

Question 8

What is the main advantage of using molecular methods (such as PCR) for measuring microbial growth?

Accepted Answer

They are fast, specific, and do not require culturing the microbes.

Question 9

Which of the following best describes the stationary phase of a bacterial growth curve?

Accepted Answer

The rate of cell growth equals the rate of cell death.

Question 10

A sample is diluted in a series of 1:10 dilutions, and 0.1 ml from the $$10^{-4}$$ dilution is plated, resulting in 65 colonies. What is the original concentration of bacteria in the sample?

Accepted Answer

$$65 	imes 10^4$ 	imes 10 = 6.5 	imes 10^6$ cells/ml

Question 11

Which of the following is NOT a direct method for measuring microbial growth?

Accepted Answer

Turbidity measurement

Question 12

Why is agar preferred over gelatin as a solidifying agent in microbiological media?

Accepted Answer

Agar is not degraded by most microbes and remains solid at incubation temperatures.

Question 13

Which type of medium would you use to increase the number of a specific microbe to detectable levels in a mixed sample?

Accepted Answer

Enrichment culture

Question 14

A microbiologist uses a spectrophotometer to measure the optical density of a bacterial culture. What is she measuring?

Accepted Answer

Turbidity, which correlates with cell density

Question 15

Which of the following best describes the purpose of transport media?

Accepted Answer

To maintain and preserve clinical specimens during transport without supporting microbial growth.

Question 16

Which of the following equations correctly describes the number of cells ($$N) $$after $$n$$ generations, starting from an initial population ($$N_0$$)?

Accepted Answer

$$N = N_0$ 	imes 2^n$$

Question 17

Which method would be most appropriate for estimating the number of coliform bacteria in a water sample when the expected concentration is low?

Accepted Answer

Most Probable Number (MPN) method

Question 18

Which of the following statements about complex media is correct?

Accepted Answer

Complex media contain ingredients of unknown chemical composition, such as peptones or extracts.

Question 19

Which of the following is a key feature of a chemostat used in industrial microbiology?

Accepted Answer

It maintains a microbial population in a steady state by continuous addition of fresh medium and removal of spent medium.

Question 20

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

Accepted Answer

Cells are adjusting to their environment and not yet dividing rapidly.

Microbial Nutrition and Growth: Culturing and Measuring Microbial Populations

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