Q: A bacterial culture starts with 100 cells. If the generation time is 30 minutes, how many cells will there be after 5 generations, assuming exponential growth and no cell death? 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.

$$N = 100 \times 2^5$ = 3,200$$ cells

Q: A sample is diluted and plated, resulting in 150 colonies on a plate from a 1:10,000 dilution of 1 mL. What is the original concentration of bacteria in the sample? Use the formula $$\text{CFU/mL} = \frac{\text{Number of colonies}}{\text{Volume plated (mL)} \times \text{Dilution factor}}.$$

$$\text{CFU/mL} = \frac{150}{1 \times 1/10,000} = 1,500,000$$

Question 1

A hospital is preparing surgical instruments for use in an operation. Which method should be used to ensure all microbial life, including endospores, is destroyed, and why?

Accepted Answer

Sterilization, because it destroys or removes all forms of microbial life, including endospores.

Question 2

A bacterial culture starts with 100 cells. If the generation time is 30 minutes, how many cells will there be after 5 generations, assuming exponential growth and no cell death? 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

$$N = 100 	imes 2^5$ = 3,200$$ cells

Question 3

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

Accepted Answer

The rate of new cell production approximately equals the rate of cell death due to nutrient limitation and waste accumulation.

Question 4

A microbiologist observes bacterial growth only at the bottom of a thioglycolate tube. Which oxygen requirement does this organism most likely have?

Accepted Answer

Obligate anaerobe

Question 5

Why can refrigerated food still spoil over time, even though most bacteria grow best at warmer temperatures?

Accepted Answer

Psychrotrophs can grow at low temperatures, including those found in refrigerators.

Question 6

Which of the following best distinguishes selective from differential media?

Accepted Answer

Selective media favor the growth of some organisms while inhibiting others; differential media reveal visible differences among organisms growing on the medium.

Question 7

A sample is diluted and plated, resulting in 150 colonies on a plate from a 1:10,000 dilution of 1 mL. What is the original concentration of bacteria in the sample? Use the formula $$	ext{CFU/mL} = \frac{	ext{Number of colonies}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution factor}}.$$

Accepted Answer

$$	ext{CFU/mL} = \frac{150}{1 	imes 1/10,000} = 1,500,000$$

Question 8

Which of the following statements about biofilms is correct?

Accepted Answer

Biofilms are communities of microorganisms attached to surfaces and embedded in a protective matrix, making them harder to remove or treat.

Question 9

Which type of culture medium would you use to isolate Gram-negative bacteria and differentiate those that ferment lactose?

Accepted Answer

MacConkey agar

Question 10

A laboratory technician needs to sterilize a heat-sensitive vitamin solution. Which method is most appropriate?

Accepted Answer

Filtration

Question 11

Why might a direct microscopic count overestimate the number of viable bacteria in a sample compared to a viable plate count?

Accepted Answer

Direct microscopic count includes both living and dead cells, while viable plate count only counts living cells capable of forming colonies.

Question 12

Which enzyme allows Helicobacter pylori to survive in the acidic environment of the stomach by neutralizing its immediate surroundings?

Accepted Answer

Urease

Question 13

A patient’s wound is being cleaned before surgery. Which term best describes the use of an antimicrobial agent on the skin to reduce microbial numbers?

Accepted Answer

Antisepsis

Question 14

Which of the following best explains why endospores are a critical consideration when choosing a decontamination method?

Accepted Answer

Endospores are highly resistant to many physical and chemical agents, requiring more rigorous sterilization methods.

Question 15

Which of the following best describes a halophile?

Accepted Answer

An organism that requires high salt concentrations for growth.

Question 16

A researcher is culturing a fastidious bacterium that requires many specific nutrients. Which type of medium is most appropriate?

Accepted Answer

Complex medium

Question 17

Which phase of the bacterial growth curve is most susceptible to antibiotics that target cell wall synthesis, and why?

Accepted Answer

Log (exponential) phase, because cells are dividing rapidly and synthesizing new cell walls.

Question 18

Which of the following best describes the function of catalase and superoxide dismutase in bacteria?

Accepted Answer

They protect cells from reactive oxygen species by breaking down toxic byproducts.

Question 19

A sample is filtered through a membrane to concentrate bacteria from 100 mL of water. The filter is then placed on an agar plate and 50 colonies grow. What is the bacterial concentration in the original sample? Use the formula $$	ext{CFU/mL} = \frac{	ext{Number of colonies}}{	ext{Volume filtered (mL)}}.$$

Accepted Answer

$$	ext{CFU/mL} = \frac{50}{100} = 0.5$$

Question 20

Which of the following best describes a microaerophile?

Accepted Answer

Requires oxygen at lower-than-atmospheric concentrations and grows as a band below the surface in a thioglycolate tube.

Fundamentals of Microbial Growth and Decontamination – Study Guide

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