Q: Sally transferred 15 Staphylococcus aureus cells to her sandwich at 8 AM. If the generation time is 20 minutes, how many cells will be present at 12 PM? (Assume exponential growth and no cell death.)

$$15 \times 2$$^{12}$$ = 61,440$$ cells

Question 1

Which of the following best explains why obligate anaerobes cannot survive in the presence of oxygen?

Accepted Answer

They lack enzymes such as superoxide dismutase and catalase needed to detoxify reactive oxygen species.

Question 2

A microbiologist wants to isolate a bacterium from ocean water. Which of the following media characteristics would best support the selective growth of this organism?

Accepted Answer

High salt concentration to mimic the marine environment.

Question 3

Which of the following best describes a mesophile?

Accepted Answer

An organism that grows optimally at temperatures between $$20^\circ C$$ and $$45^\circ C.$$

Question 4

Blood agar is best classified as which of the following types of media?

Accepted Answer

Enriched and differential

Question 5

A fastidious, Gram-negative, obligate anaerobic mesophile would require which of the following conditions for optimal growth?

Accepted Answer

Enriched, oxygen-free medium at $$37^\circ C$$

Question 6

Which of the following best describes a biofilm?

Accepted Answer

A complex community of microorganisms attached to a surface and embedded in a self-produced extracellular matrix.

Question 7

Why do bacterial cultures need to be transferred to fresh media regularly?

Accepted Answer

To replenish nutrients and prevent accumulation of toxic metabolic byproducts.

Question 8

Which of the following best describes the process of obtaining a pure culture from a mixed specimen?

Accepted Answer

Streaking the specimen on an agar plate to isolate single colonies, then transferring a single colony to fresh medium.

Question 9

A student transfers 1 ml of a culture broth containing Escherichia coli to 9 ml of sterile broth, then transfers 1 ml from this dilution to a culture plate. If 42 colonies grow on the plate, what was the concentration per ml in the original culture broth?

Accepted Answer

420 cells/ml

Question 10

Sally transferred 15 Staphylococcus aureus cells to her sandwich at 8 AM. If the generation time is 20 minutes, how many cells will be present at 12 PM? (Assume exponential growth and no cell death.)

Accepted Answer

$$15 	imes 2$$^{12}$$ = 61,440$$ cells

Question 11

Which of the following best describes an autotroph?

Accepted Answer

An organism that uses $$CO_2 as $$its carbon source.

Question 12

Human pathogens are most accurately classified as which of the following?

Accepted Answer

Heterotrophs and mesophiles

Question 13

Which of the following best describes a selective medium?

Accepted Answer

A medium that inhibits the growth of some organisms while allowing others to grow.

Question 14

Which of the following would be the best medium to selectively grow a bacterium that lives in the stomach?

Accepted Answer

A medium with low pH to mimic stomach acid.

Question 15

Which of the following statements about phototrophs is correct?

Accepted Answer

They use light as their energy source.

Question 16

A halophilic, aerobic thermophile would require which of the following conditions for optimal growth?

Accepted Answer

High salt concentration, oxygen, and elevated temperature (above $$45^\circ C)$$

Question 17

Which of the following best explains why differential media are useful in microbiology?

Accepted Answer

They allow the distinction between different types of bacteria based on observable changes, such as color or hemolysis.

Question 18

Which of the following is a key biochemical reason why aerotolerant anaerobes can survive in the presence of oxygen?

Accepted Answer

They possess superoxide dismutase but lack catalase.

Question 19

Which of the following best describes the difference between enriched and selective media?

Accepted Answer

Enriched media provide extra nutrients for fastidious organisms, while selective media inhibit the growth of certain organisms.

Microbial Growth: Requirements, Classification, and Laboratory Techniques

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