Q: A bacterial culture starts with 100 cells. If the generation time is 30 minutes and the culture grows for 2.5 hours, how many cells will there be at the end? 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.

$$N = 100 \times 2^5$ = 3200$$ cells

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 and the culture grows for 2.5 hours, how many cells will there be at the end? 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

$$N = 100 	imes 2^5$ = 3200$$ cells

Question 3

Which of the following best describes the difference between selective and differential media, using MacConkey agar as an example?

Accepted Answer

Selective media only allows certain organisms to grow, while differential media shows visible differences among organisms; MacConkey agar is both selective for Gram-negative bacteria and differential for lactose fermentation.

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 refrigeration slows microbial growth?

Accepted Answer

Refrigeration only kills mesophiles, but psychrotrophs and psychrophiles can still grow and cause spoilage.

Question 6

Which phase of the bacterial growth curve is characterized by a balance between new cell production and cell death due to nutrient limitation and waste accumulation?

Accepted Answer

Stationary phase

Question 7

A laboratory technician needs to count only viable bacteria in a water sample. Which method should they use and why?

Accepted Answer

Viable plate count, because it counts only colony-forming units representing living cells.

Question 8

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

Accepted Answer

Endospores are highly resistant to heat, chemicals, and radiation, so only sterilization methods can reliably destroy them.

Question 9

A patient is infected with Helicobacter pylori, which survives in the acidic environment of the stomach. Which adaptation allows it to persist there?

Accepted Answer

Production of urease to neutralize its immediate environment.

Question 10

Which of the following best describes a complex culture medium?

Accepted Answer

A medium containing ingredients with variable or not fully defined chemical composition.

Question 11

A scientist is culturing a halophilic bacterium. Which of the following conditions should be provided for optimal growth?

Accepted Answer

High salt concentration

Question 12

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 13

A culture medium contains peptone, beef extract, and agar, but the exact chemical composition is not known. Which type of medium is this?

Accepted Answer

Complex medium

Question 14

Which of the following best explains why catalase and superoxide dismutase are important for some bacteria?

Accepted Answer

They protect cells from reactive oxygen species, allowing survival in oxygen-rich environments.

Question 15

A sample is diluted 1:1000 and 0.1 mL is plated, resulting in 50 colonies. 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

$$	ext{CFU/mL} = \frac{50}{0.1 	imes 0.001} = 500,000$$

Question 16

Which of the following best describes the main difference between sterilization and disinfection?

Accepted Answer

Sterilization removes all forms of microbial life, including endospores, while disinfection reduces or destroys many pathogens but not necessarily endospores.

Question 17

A researcher wants to isolate a Gram-positive bacterium from a mixed sample. Which medium would be most appropriate and why?

Accepted Answer

CNA agar, because it inhibits Gram-negative bacteria and selects for Gram-positive bacteria.

Question 18

Which of the following best explains why colony-forming units (CFUs) and direct microscopic cell counts can give different results?

Accepted Answer

CFUs count only living cells capable of forming colonies, while direct microscopic counts include both living and dead cells.

Question 19

A laboratory is testing a new hand sanitizer. Which term best describes the use of this product on skin to reduce microbial load?

Accepted Answer

Antisepsis

Question 20

Which of the following best describes a facultative anaerobe's growth pattern in a thioglycolate tube?

Accepted Answer

Growth throughout the tube, but more concentrated at the top.

Fundamentals of Microbial Growth and Decontamination – Study Guide

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