Question 1

During a Gram stain procedure, what would be the appearance of a bacterial smear containing both Gram-positive and Gram-negative organisms if the student forgets to add the counterstain (safranin)?

Accepted Answer

Gram-positive organisms would appear purple, and Gram-negative organisms would be colorless.

Question 2

Which step in the Gram stain procedure is responsible for differentiating between Gram-positive and Gram-negative bacteria?

Accepted Answer

Decolorization with alcohol or acetone

Question 3

A sputum smear is stained using the acid-fast technique. You observe red, rod-shaped cells under the microscope. What does this indicate?

Accepted Answer

The presence of acid-fast bacteria, such as Mycobacterium species

Question 4

In the capsule stain using India ink, how are bacterial capsules visualized under the microscope?

Accepted Answer

As clear halos surrounding the cells

Question 5

Which of the following is the correct order of reagents in the Gram stain procedure?

Accepted Answer

Crystal violet → iodine → decolorizer → safranin

Question 6

Why is heat fixation of the smear performed before staining in both Gram and acid-fast stain procedures?

Accepted Answer

To attach the cells to the slide and prevent them from washing off during staining

Question 7

What is the function of the counterstain in the Gram stain procedure?

Accepted Answer

To make Gram-negative cells visible

Question 8

A student performs a streak-plate isolation and observes that the sample from the first technician produces colonies with different morphologies, while a fresh patient sample produces only one type of colony. What is the most likely explanation?

Accepted Answer

The first technician's sample was contaminated with multiple types of bacteria.

Question 9

In the acid-fast stain, what is the role of acid-alcohol?

Accepted Answer

To remove the primary stain from bacteria that do not have waxy cell walls

Question 10

Which of the following best describes the appearance of non-acid-fast bacteria after an acid-fast stain?

Accepted Answer

Blue due to the counterstain with methylene blue

Question 11

What is the primary stain used in the endospore staining procedure?

Accepted Answer

Malachite green

Question 12

Which of the following media is both differential and selective, commonly used to isolate and identify Staphylococcus aureus?

Accepted Answer

Mannitol salt agar

Question 13

A bacterium is tested for catalase activity by adding hydrogen peroxide. Bubbling is observed. What does this indicate?

Accepted Answer

The bacterium is catalase positive and produces oxygen gas.

Question 14

What is the expected result of a starch hydrolysis test if the bacterium produces amylase?

Accepted Answer

A clear zone around the growth after adding Gram's iodine

Question 15

Which of the following is a correct interpretation of a Triple Sugar Iron (TSI) agar test showing a red slant and yellow butt?

Accepted Answer

Glucose fermentation only

Question 16

What is the main reason acid-fast bacteria retain the primary stain carbolfuchsin after treatment with acid-alcohol?

Accepted Answer

They have waxy material (mycolic acids) in their cell walls.

Question 17

Which of the following best describes the charge interaction that allows basic dyes to bind to bacterial cells?

Accepted Answer

The positive charge of the dye adheres to the negative charge of the bacterial cell surface.

Question 18

Which of the following is a selective medium that inhibits the growth of most bacteria except staphylococci?

Accepted Answer

Mannitol salt agar

Question 19

A microbiologist observes blue cells after performing an acid-fast stain. What does this indicate about the cells?

Accepted Answer

They are non-acid-fast bacteria.

Microbiology Study Notes: Bacterial Staining, Isolation, and Biochemical Identification

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