Q: A culture of E. coli is grown for 2 hours, and the generation time is 20 minutes. How many generations have occurred?

$$n = \frac{120\ \text{minutes}}{20\ \text{minutes/generation}} = 6$$

Question 1

Which of the following best describes the difference between microbial growth in a laboratory pure culture and in nature?

Accepted Answer

In the laboratory, microbes are typically grown as single-species cultures under controlled conditions, while in nature they interact with other species and environmental factors.

Question 2

A bacterium divides by binary fission every 30 minutes. If you start with 1 cell, how many cells will there be after 3 hours? Use the formula $$N = N_0$ 	imes 2^n$$, where $$n is $$the number of generations.

Accepted Answer

$$N = 1 	imes 2^6$ = 64$$

Question 3

Which phase of the bacterial growth curve is characterized by a balance between cell division and cell death?

Accepted Answer

Stationary phase

Question 4

Which of the following statements about bacterial endospores is correct?

Accepted Answer

Endospores are thick-walled, nongrowing structures that allow bacteria to survive harsh conditions.

Question 5

A culture of E. coli is grown for 2 hours, and the generation time is 20 minutes. How many generations have occurred?

Accepted Answer

$$n = \frac{120\ 	ext{minutes}}{20\ 	ext{minutes/generation}} = 6$$

Question 6

Which group of microbes is most likely to cause human disease based on their optimal temperature range?

Accepted Answer

Mesophiles

Question 7

Which of the following best describes a facultative anaerobe?

Accepted Answer

Can grow with or without oxygen and can switch between aerobic respiration and fermentation.

Question 8

Which of the following media would be most appropriate for isolating and differentiating Staphylococcus aureus from other bacteria in a clinical sample?

Accepted Answer

Mannitol salt agar (MSA)

Question 9

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

Accepted Answer

Viable plate count

Question 10

Which of the following statements about sterilization and disinfection is correct?

Accepted Answer

Sterilization eliminates all microbes, including endospores, while disinfection reduces microbial numbers but may not eliminate all pathogens.

Question 11

Which physical method is most effective for ensuring the elimination of bacterial endospores from laboratory glassware?

Accepted Answer

Autoclaving at 121\degree C and 15 psi for 20 minutes

Question 12

Which of the following best explains why UV light is used to disinfect surfaces but not to sterilize packaged medical equipment?

Accepted Answer

UV light cannot penetrate solid barriers such as packaging.

Question 13

Which of the following is a correct pairing of a germicide and its typical use?

Accepted Answer

Aldehydes – sterilize surgical instruments

Question 14

A patient sample is suspected to contain a fastidious bacterium. Which type of media should be used to ensure its growth?

Accepted Answer

Complex (enriched) media

Question 15

Which of the following best describes the action of surfactants in detergents?

Accepted Answer

They physically bind to and disrupt lipid membranes, increasing permeability.

Question 16

Which of the following is a real-world application of the streak plate technique?

Accepted Answer

To isolate a pure culture from a mixed clinical sample.

Question 17

Which of the following best explains why Mycobacterium tuberculosis is difficult to control with standard disinfectants?

Accepted Answer

Its cell wall contains waxy mycolic acids, making it resistant to many disinfectants.

Question 18

Which of the following is the best method for sterilizing heat-sensitive medical equipment such as plastic syringes?

Accepted Answer

Ethylene oxide gas

Question 19

A laboratory technician needs to differentiate between Gram-positive and Gram-negative bacteria in a stool sample. Which medium would be most useful?

Accepted Answer

Eosin methylene blue agar (EMB)

Question 20

Which of the following best describes the main advantage of genotypic methods (such as PCR) over phenotypic methods for microbial identification?

Accepted Answer

Genotypic methods are faster and can identify viable noncultured microbes.

Fundamentals of Microbial Growth and Decontamination: Study Notes

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