Question 1

Which of the following environmental factors is LEAST likely to affect the morphology of a bacterial colony grown on an agar plate?

Accepted Answer

Atmospheric pressure

Question 2

A student observes a bacterial colony that is large, round, opaque, with a smooth surface, moist texture, and raised elevation. Which of the following best describes the margin of the colony if it appears wavy at the edges?

Accepted Answer

Undulate

Question 3

During a Gram stain, what is the primary function of the iodine step?

Accepted Answer

To act as a mordant and form a complex with crystal violet

Question 4

If a student skips the iodine step during Gram staining, what is the most likely result?

Accepted Answer

Gram positive cells may appear Gram negative

Question 5

Which of the following best explains why Gram negative bacteria lose the crystal violet stain during the decolorization step?

Accepted Answer

Alcohol removes lipids from the outer membrane, creating holes that allow the stain to escape

Question 6

A microbiologist performs a Gram stain but leaves the alcohol decolorization step for too long. What is the most likely appearance of the cells under the microscope?

Accepted Answer

Gram positive cells appear reddish, and Gram negative cells appear pink

Question 7

Which of the following is NOT a correct pairing of colony morphology term and its category?

Accepted Answer

Dull – Shape

Question 8

A bacterial colony is described as 'filamentous' at its margin. What does this indicate about the colony's appearance?

Accepted Answer

It has edges with unbranched, thread-like strands

Question 9

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

Accepted Answer

Decolorization with alcohol

Question 10

A student observes concentric rings of bacterial growth radiating from a central point on an agar plate. What phenomenon is most likely being observed?

Accepted Answer

Swarming

Question 11

If the decolorization step is skipped during Gram staining, what is the expected result?

Accepted Answer

All cells will appear purple

Question 12

Which of the following best describes the chemical interaction that occurs when iodine is added during the Gram stain?

Accepted Answer

Iodine forms a complex with crystal violet, making it less soluble

Question 13

A clinical sample is Gram stained and all cells appear purple under the microscope. Which of the following procedural errors could explain this result?

Accepted Answer

Decolorization step was skipped

Question 14

Which of the following best explains why Gram positive bacteria retain the crystal violet-iodine complex after decolorization?

Accepted Answer

Their thick peptidoglycan layer becomes dehydrated, trapping the complex inside

Question 15

A microbiologist describes a colony as 'umbonate.' To which category of colony morphology does this term belong?

Accepted Answer

Elevation

Question 16

Which of the following is the correct sequence of steps in the Gram stain procedure?

Accepted Answer

Crystal violet → Iodine → Alcohol → Safranin

Question 17

A patient sample is Gram stained and the cells appear pink under the microscope. Which of the following best explains this result?

Accepted Answer

The cells are Gram negative and have lost the crystal violet stain during decolorization

Question 18

Which of the following best describes the effect of insufficient decolorization during the Gram stain procedure?

Accepted Answer

Gram negative cells may appear purple

Question 19

A researcher is comparing two bacterial colonies. Colony A is described as 'butyrous' and Colony B as 'dry.' To which category of colony morphology do these terms belong?

Accepted Answer

Texture

Question 20

Which of the following best describes the appearance of a 'pulvinate' colony?

Accepted Answer

A colony with a very convex, dome-shaped elevation

Microbiology Lab Study Guide: Colony Morphology and Gram Staining

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