Question 1

A microbiologist is studying bacteria that spoil refrigerated food. Which group of bacteria is most likely responsible for this spoilage, and what is their optimum growth temperature?

Accepted Answer

Psychrotrophs; optimum growth between $$20^\circ C$$ and $$30^\circ C$$

Question 2

Which of the following best explains why most bacteria cannot survive in a jar of honey?

Accepted Answer

The high osmotic pressure causes plasmolysis due to water loss from bacterial cells.

Question 3

A bacterium isolated from a hot spring grows optimally at $$55^\circ C. $$Which term best describes this organism?

Accepted Answer

Thermophile

Question 4

Which of the following statements about acidophiles is correct?

Accepted Answer

They thrive in acidic environments, often at pH values below 5.

Question 5

A laboratory technician is preparing a medium for a fastidious organism. Which type of medium should be used and why?

Accepted Answer

Chemically defined medium, because it provides precise nutrients required by the organism.

Question 6

Which of the following best describes the function of superoxide dismutase (SOD) in aerobic bacteria?

Accepted Answer

It removes superoxide radicals by catalyzing the reaction: $$2O_2^- + 2H^+ \rightarrow H_2O_2 + O_2$

Question 7

A patient sample is suspected to contain an obligate anaerobe. Which culture technique would be most appropriate to isolate this organism?

Accepted Answer

Use of reducing media containing sodium thioglycolate

Question 8

Which of the following best explains why agar is commonly used as a solidifying agent in microbiological media?

Accepted Answer

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

Question 9

A researcher wants to isolate Salmonella Typhi from a fecal sample containing many types of bacteria. Which type of medium should be used and why?

Accepted Answer

Selective medium, to suppress unwanted microbes and encourage Salmonella Typhi growth.

Question 10

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

Accepted Answer

The number of new cells produced equals the number of cells dying.

Question 11

A microbiologist wants to determine the number of viable bacteria in a water sample. Which direct method would be most appropriate?

Accepted Answer

Plate count after serial dilution

Question 12

Which of the following equations correctly represents the calculation of the number of bacteria per milliliter using a direct microscopic count?

Accepted Answer

$$	ext{Number of bacteria/mL} = \frac{	ext{Number of cells counted}}{	ext{Volume of area counted}}$$

Question 13

A culture medium contains yeast extract, peptone, and glucose. Which type of medium is this and why?

Accepted Answer

Complex, because it contains extracts and digests of natural products.

Question 14

Which of the following best describes the function of catalase in bacteria?

Accepted Answer

It converts hydrogen peroxide to water and oxygen: $$2H_2O_2 \rightarrow 2H_2O + O_2$

Question 15

A bacterial culture is growing in a medium with a starting population of 1,000 cells. If the generation time is 30 minutes, how many cells will there be after 2 hours of exponential growth?

Accepted Answer

$$1,000 	imes 2^4$ = 16,000$$ cells

Question 16

Which of the following best explains why buffers are added to growth media in the laboratory?

Accepted Answer

To minimize pH changes caused by microbial metabolism.

Question 17

Which biosafety level (BSL) is required for working with highly infectious airborne pathogens such as Mycobacterium tuberculosis?

Accepted Answer

BSL-3

Question 18

A scientist is using the most probable number (MPN) method to estimate bacterial numbers in a water sample. Which of the following best describes this method?

Accepted Answer

It uses a statistical table to estimate cell numbers based on positive growth in multiple tubes.

Question 19

Which of the following is a correct statement about facultative anaerobes?

Accepted Answer

They can grow with or without oxygen, using fermentation or anaerobic respiration when oxygen is absent.

Question 20

Which of the following is a real-world application of enrichment culture?

Accepted Answer

Isolating a rare bacterium from a soil sample by providing specific nutrients that favor its growth.

Microbial Growth: Physical and Chemical Requirements

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