Question 1

Which of the following best describes the purpose of the aseptic technique in microbiology laboratories?

Accepted Answer

To exclude contaminants and prevent unwanted microorganisms from entering cultures.

Question 2

A microbiologist wants to isolate a pure culture from a mixed bacterial sample. Which technique should they use and why?

Accepted Answer

Streak plate technique, because it separates individual bacteria on solid media to form isolated colonies.

Question 3

Which of the following media would be most appropriate for isolating and differentiating Gram-negative enteric bacteria based on lactose fermentation?

Accepted Answer

MacConkey agar

Question 4

A student performs a 10-fold serial dilution of a bacterial culture and plates 0.1 mL from the $$10^{-4}$$ dilution. After incubation, 80 colonies are counted. What is the CFU/mL in the original sample?

Accepted Answer

$$8 	imes 10$$^{6}$$ CFU/mL

Question 5

Which of the following statements about the spread plate and pour plate methods is correct?

Accepted Answer

Pour plate method is better for counting bacteria that prefer low oxygen environments.

Question 6

On XLD agar, which result would indicate the presence of Salmonella species?

Accepted Answer

Red colonies with black centers

Question 7

Why is CLED agar particularly useful for urine cultures?

Accepted Answer

It prevents swarming of Proteus species and differentiates lactose fermenters from non-fermenters.

Question 8

A microbiologist observes yellow colonies on TCBS agar after inoculating a stool sample. What is the most likely organism present?

Accepted Answer

Vibrio cholerae

Question 9

Which of the following best explains why agar deeps are used in microbiology?

Accepted Answer

To grow bacteria that require less oxygen and to check for motility.

Question 10

Which of the following is a critical step in the aseptic transfer of bacteria to prevent contamination?

Accepted Answer

Flaming the mouth of the tube before and after transfer.

Question 11

A student streaks a mixed culture on a Petri plate and observes isolated colonies after incubation. What does each isolated colony most likely represent?

Accepted Answer

A single bacterium or a group of the same bacteria that multiplied.

Question 12

Which of the following best describes the function of selective media?

Accepted Answer

It inhibits the growth of unwanted microorganisms.

Question 13

Which of the following is the correct sequence for performing a 10-fold serial dilution?

Accepted Answer

Add 1 mL of sample to 9 mL sterile water, mix, and repeat with each subsequent dilution.

Question 14

Which of the following best explains why the mouth of a culture tube is passed through a flame during aseptic technique?

Accepted Answer

To create an upward air current that prevents airborne contaminants from entering.

Question 15

A microbiologist is using XLD agar and observes red colonies without black centers. What is the most likely organism?

Accepted Answer

Shigella species

Question 16

Which of the following best describes the role of bromothymol blue in CLED agar?

Accepted Answer

It acts as a pH indicator to differentiate lactose fermenters (yellow) from non-fermenters (blue/translucent).

Question 17

Which of the following media would be most appropriate for growing obligate anaerobes?

Accepted Answer

Thioglycolate broth

Question 18

A student is asked to identify a lactose-fermenting bacterium on MacConkey agar. What colony color should they look for?

Accepted Answer

Pink

Question 19

Which of the following best explains why the inoculating loop must be cooled before touching the bacterial culture?

Accepted Answer

To prevent killing the bacteria with heat.

Question 20

Which of the following best describes the growth pattern of facultative anaerobes in thioglycolate broth?

Accepted Answer

Growth throughout the medium, but denser at the top.

Aseptic Technique, Bacterial Culture Methods, and Differential Media in Microbiology

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