Question 1

Which step in the Gram stain procedure is most critical for differentiating between Gram-positive and Gram-negative bacteria, and what is the main consequence if this step is performed incorrectly?

Accepted Answer

Alcohol wash (decolorization); it may cause Gram-positive cells to appear Gram-negative if overapplied.

Question 2

Refer to the image showing three bacterial shapes and Gram stain results. Which of the following correctly matches the shape and Gram reaction for the labeled 'Rod' and 'Coccus' cells?

Accepted Answer

'Rod' is Gram-negative, 'Coccus' is Gram-positive

Question 3

In the endospore stain, which combination of stains and cell types is correct?

Accepted Answer

Malachite green stains endospores; safranin stains vegetative cells.

Question 4

Which of the following best describes the function of the decolorizer in the Gram stain procedure?

Accepted Answer

To remove the color from Gram-negative bacteria

Question 5

A student performs a Gram stain on a bacterial culture that is 48 hours old. The Gram-positive cells appear pink. What is the most likely explanation?

Accepted Answer

The culture is too old, and the cell walls have degraded.

Question 6

Which of the following is a selective and differential medium for isolating Staphylococcus aureus, and what is the indicator used?

Accepted Answer

Mannitol Salt Agar; phenol red

Question 7

On a Mannitol Salt Agar plate, a yellow zone appears around the colonies. What does this indicate?

Accepted Answer

The bacteria are fermenting mannitol, producing acid.

Question 8

A blood agar plate shows a clear, colorless zone around the colonies. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells

Question 9

Which of the following statements about the catalase test is correct?

Accepted Answer

A positive test is indicated by the production of bubbles when hydrogen peroxide is added.

Question 10

In the DNAse test, what result indicates a positive test and what is the indicator used?

Accepted Answer

A clear zone around colonies; methyl green

Question 11

Which of the following best describes the result of a positive citrate test?

Accepted Answer

The medium turns blue due to an alkaline pH, indicating citrate utilization.

Question 12

In the SIM test, which indicator is used to detect indole production, and what is a positive result?

Accepted Answer

Kovac's reagent; a red ring at the top of the tube

Question 13

Which of the following is NOT an acid/base indicator used in microbiological media?

Accepted Answer

Kovac's reagent

Question 14

Which chemical agent is classified as an oxidizing agent and is commonly used for disinfection?

Accepted Answer

Hydrogen peroxide

Question 15

A water sample is tested for coliforms using the multitube fermentation test. Which step involves performing a Gram stain to confirm the presence of coliforms?

Accepted Answer

Completed test

Question 16

On Eosin Methylene Blue (EMB) agar, which result indicates a strong lactose fermenter such as Escherichia coli?

Accepted Answer

Colonies with a metallic green sheen

Question 17

Which of the following best describes the Kirby-Bauer disk diffusion method for testing antimicrobial susceptibility?

Accepted Answer

It measures the zone of inhibition around antibiotic disks on a Mueller-Hinton plate.

Question 18

Which of the following is a characteristic of an ideal antimicrobial agent?

Accepted Answer

Selectively toxic against a wide range of pathogens

Question 19

In the urease test, what is the chemical reaction that leads to a positive result, and what color change is observed?

Accepted Answer

Urea is hydrolyzed to ammonia, causing the medium to turn pink.

Question 20

Which of the following best defines a coliform in water analysis?

Accepted Answer

A Gram-negative, lactose-fermenting, rod-shaped bacterium that produces gas at 35°C

Lab Review 7- 12: Differential Stains, Biochemical Tests, and Microbial Control

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