Question 1

Which of the following best describes the main difference between preparing a bacterial smear from a solid medium versus a liquid medium?

Accepted Answer

A smear from a solid medium requires adding a drop of water to the slide before transferring bacteria, while a smear from a liquid medium does not.

Question 2

During the Gram-staining procedure, what is the primary function of the mordant (iodine)?

Accepted Answer

To form a complex with the crystal violet, making it less soluble and trapping it in the thick peptidoglycan layer of Gram-positive bacteria.

Question 3

After completing a Gram stain, you observe purple cocci in clusters under the microscope. What can you conclude about the bacteria?

Accepted Answer

They are Gram-positive cocci, likely with a thick peptidoglycan layer.

Question 4

Which of the following best describes a bactericidal agent?

Accepted Answer

An agent that kills bacteria, resulting in a decrease in viable cell count.

Question 5

A student performs a use-dilution test to evaluate a new disinfectant. Which of the following best describes the purpose of this test?

Accepted Answer

To determine the effectiveness of a disinfectant at different concentrations against a standard bacterial population.

Question 6

In the Kirby-Bauer disk diffusion test, what does a large zone of inhibition around an antibiotic disk indicate?

Accepted Answer

The bacteria are highly sensitive to the antibiotic.

Question 7

Which of the following statements about Gram-negative bacteria is correct?

Accepted Answer

They have a thin peptidoglycan layer and appear pink after Gram staining.

Question 8

A microbiologist wants to determine the decimal reduction time (DRT) for a disinfectant. Which equation should they use to calculate the DRT, given the initial and final bacterial populations and the time of exposure?

Accepted Answer

$$ D = \frac{t}{\log_{10}(N_0$) - \log_{10}(N)} $$

Question 9

Which of the following best describes the purpose of heat-fixing a bacterial smear before staining?

Accepted Answer

To kill the bacteria and adhere them to the slide, preventing them from washing off during staining.

Question 10

A patient sample is tested using the Kirby-Bauer method on Mueller-Hinton agar. The zone of inhibition for antibiotic X is measured at 18 mm. According to CLSI standards, resistance is defined as a zone less than 15 mm. What is the correct interpretation?

Accepted Answer

The bacteria are sensitive to antibiotic X.

Question 11

Which of the following is a correct pairing of bacterial morphology and arrangement?

Accepted Answer

Streptobacilli: rod-shaped bacteria arranged in chains

Question 12

When focusing a specimen under a compound light microscope, which sequence of steps is most appropriate?

Accepted Answer

Start with the lowest power objective, use coarse focus, then switch to higher power and use fine focus.

Question 13

Which of the following statements about antiseptics and disinfectants is correct?

Accepted Answer

Antiseptics are safe for use on living tissue, while disinfectants are used on inanimate objects.

Question 14

During Gram staining, what is the role of the decolorizing agent (usually alcohol or acetone)?

Accepted Answer

To remove the crystal violet-iodine complex from Gram-negative cells, making them colorless before counterstaining.

Question 15

A wet mount preparation is most useful for which of the following observations?

Accepted Answer

Observing live cell motility and natural cell arrangement.

Question 16

Which of the following best explains why Gram-positive bacteria retain the primary stain after the decolorization step?

Accepted Answer

Their thick peptidoglycan layer traps the crystal violet-iodine complex.

Question 17

A hospital is testing a new hand sanitizer. They perform a use-dilution test and find that after 10 minutes, the number of viable bacteria drops from $$10^6 to$$ $$10^3. $$What is the decimal reduction time (DRT) for this sanitizer?

Accepted Answer

$$ D = \frac{10}{\log_{10}(10^6$) - \log_{10}(10^3$)} = \frac{10}{6-3} = \frac{10}{3} \approx 3.33 	ext{ min} $$

Question 18

Which of the following is NOT a correct statement about the Kirby-Bauer disk diffusion method?

Accepted Answer

It determines the minimum bactericidal concentration (MBC) of an antibiotic.

Question 19

Which of the following best describes a bacteriostatic agent?

Accepted Answer

An agent that inhibits bacterial growth but does not kill the cells.

Question 20

Which of the following is the correct sequence of reagents used in the Gram-staining procedure?

Accepted Answer

Crystal violet, iodine, alcohol/acetone, safranin

Microbiology Laboratory Techniques and Microbial Control

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