Why do Gram-positive bacteria retain the crystal violet stain?
Their outer membrane traps the stain
Their thick peptidoglycan layer holds the crystal violet-iodine complex
They absorb the safranin more strongly
They do not undergo decolorization
The Correct Answer is B
A. Their outer membrane traps the stain: Gram-positive bacteria do not have an outer membrane; this structure is characteristic of Gram-negative bacteria. Therefore, the outer membrane cannot be responsible for retaining crystal violet in Gram-positive cells.
B. Their thick peptidoglycan layer holds the crystal violet-iodine complex: Gram-positive bacteria have a thick, multilayered peptidoglycan cell wall that forms a dense network. During Gram staining, the crystal violet combines with iodine to form an insoluble complex that gets trapped within this thick peptidoglycan matrix. Even after alcohol or acetone decolorization, the complex remains, causing the cells to appear purple under a microscope.
C. They absorb the safranin more strongly: Safranin is a counterstain used to color Gram-negative bacteria red after decolorization. Gram-positive bacteria do not absorb safranin strongly because the retained crystal violet-iodine complex masks it. Therefore, safranin absorption is not responsible for the purple color of Gram-positive cells.
D. They do not undergo decolorization: While Gram-positive bacteria resist decolorization, this is not because they fail to undergo the process; it is due to the structural ability of their thick peptidoglycan to trap the crystal violet-iodine complex. The resistance is a result of the cell wall structure, not a lack of exposure to the decolorizing agent.
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Related Questions
Correct Answer is C
Explanation
A. Purple: Purple indicates Gram-positive bacteria, which retain the crystal violet-iodine complex within their thick peptidoglycan cell walls during the Gram staining process. The color persists even after decolorization with alcohol or acetone.
B. Blue: Blue is not a standard color outcome in Gram staining. Gram-negative and Gram-positive bacteria are typically differentiated as purple or pink/red; blue may appear in other specialized staining techniques but not standard Gram staining.
C. Pink/Red: Gram-negative bacteria appear pink or red after Gram staining. Their thin peptidoglycan layer does not retain the crystal violet-iodine complex after decolorization. They are counterstained with safranin, which imparts the pink/red color, allowing differentiation from Gram-positive bacteria.
D. Green: Green is not a result of Gram staining. Green coloration is associated with other staining methods or pigments but is not part of the Gram stain differential process.
Correct Answer is A
Explanation
A. To prevent distortion of cells: Allowing the smear to air-dry completely ensures that the bacterial cells retain their natural shape and size. If the smear is heat-fixed while still wet, the water can rapidly boil, causing cells to rupture, shrink, or burst, which distorts their morphology. Maintaining cell integrity is essential for accurate microscopic examination and identification of bacteria.
B. To speed up staining: Air-drying is not intended to speed up staining; in fact, it is a preparatory step to preserve cell structure before applying stains. Proper drying ensures that stains adhere evenly without artifacts caused by residual moisture.
C. To make the bacteria grow: Smear preparation does not promote bacterial growth. Heat fixing kills the bacteria, making them safe to handle and fixing them to the slide. Growth occurs only on appropriate culture media, not on a prepared smear.
D. To increase contrast: Contrast is primarily achieved through the staining process, such as Gram staining or simple stains. Air-drying itself does not enhance contrast; it only prepares the cells to be fixed and stained properly without distortion.
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