Question 1

Which of the following best describes the purpose of the zone of inhibition observed around antibiotic disks in the disk diffusion assay, as shown in the image?

Accepted Answer

It shows the region where the antibiotic has diffused and prevented bacterial growth, indicating sensitivity.

Question 2

In the disk diffusion assay, why might a larger zone of inhibition not always indicate greater effectiveness of an antibiotic when comparing different antibiotics on the same organism?

Accepted Answer

Because the molecular weight of the antibiotic affects its rate of diffusion through the agar.

Question 3

If E. coli is resistant to penicillin (P) as shown in the image, what would you expect to see around the penicillin disk?

Accepted Answer

No zone of inhibition; bacterial growth up to the disk.

Question 4

Which equation best describes the relationship between the concentration of an antibiotic and its distance from the disk in the agar?

Accepted Answer

$$C = C_0$ e$$^{-k r}$$

Question 5

What is the primary reason for using Mueller-Hinton agar in the disk diffusion assay?

Accepted Answer

It allows for consistent diffusion of antibiotics due to its standardized composition.

Question 6

If an antibiotic disk produces a zone of inhibition with a diameter of 20 mm, and another produces a zone of 10 mm, what additional information is needed to determine which antibiotic is more effective against the organism?

Accepted Answer

The molecular weight and diffusion rate of each antibiotic.

Question 7

What would you conclude if a zone of inhibition is present around the chloramphenicol (C) disk but not around the penicillin (P) disk for E. coli?

Accepted Answer

E. coli is sensitive to chloramphenicol and resistant to penicillin.

Question 8

Which factor does NOT affect the size of the zone of inhibition in the disk diffusion assay?

Accepted Answer

The color of the antibiotic disk.

Question 9

If a bacterium is classified as 'sensitive' to an antibiotic, what is the expected result on the plate?

Accepted Answer

A zone of inhibition forms around the antibiotic disk.

Question 10

Why is it important to standardize the thickness of the agar in the disk diffusion assay?

Accepted Answer

To ensure consistent diffusion rates of antibiotics.

Microbiology Practical Review – Media, Indicators, and Antimicrobial Susceptibility

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