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The Organisms Gram Staining Misses Quiz
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What does a routine Gram stain primarily test about a bacterial cell?
- A Whether the cell wall has a thick peptidoglycan layer that prevents counterstain uptake You picked ✕
- B Whether the cell contains endospores that resist routine staining You picked ✕
- C Whether long-chain mycolic acids keep carbolfuchsin after acid-alcohol You picked ✕
- D Whether the cell wall contains an outer membrane that prevents the primary stain from entering You picked ✕
- E Whether the envelope retains crystal-violet-iodine during decolourisation You picked ✓
Correct!
Incorrect
The answer is E: Whether the envelope retains crystal-violet-iodine during decolourisation
Explanation
Why E is correct
See When Gram Staining Is Unhelpful. Option E is correct because a Gram stain asks one structural question: after crystal violet (a purple dye) and iodine have formed a complex inside the cell, does the cell keep that complex when alcohol or acetone is used to wash the slide? This washing step is called decolourisation. The cell envelope (the membrane and the wall around it) contains peptidoglycan, a mesh of sugars and short peptides that strengthens the wall. A thick peptidoglycan layer traps the complex, so the cell stays purple and is called Gram-positive. A thin layer lying behind an outer membrane lets the dye wash out, so the cell is colourless until a pink counterstain (a second dye, usually safranin) is added, and it is called Gram-negative. The result sorts bacteria by how their envelope behaves; it does not name the species, and some bacteria, such as mycobacteria and Mycoplasma, give no reliable Gram result at all. Memory aid: purple holds on, pink lets go.
- Test: a Gram stain asks whether the cell keeps the crystal violet and iodine complex after alcohol or acetone.
- Thick peptidoglycan: traps the complex, so the cell stays purple and is Gram-positive.
- Thin layer: behind an outer membrane lets the dye wash out, so the cell is pink after the counterstain and is Gram-negative.
- Limit: the result sorts bacteria by envelope behaviour and does not name the species.
Why the other options are incorrect
- A. Whether the cell wall has a thick peptidoglycan layer that prevents counterstain uptake: The counterstain is taken up by cells regardless of peptidoglycan thickness; Gram-positive cells remain purple because they retain the primary stain, not because the wall blocks safranin. This option confuses retention of the primary stain with exclusion of the counterstain. Your answer
- B. Whether the cell contains endospores that resist routine staining: Endospores are tough, dormant survival structures made inside a few kinds of bacteria. They stay clear on a Gram film, so a dedicated spore stain is needed to show them. Your answer
- C. Whether long-chain mycolic acids keep carbolfuchsin after acid-alcohol: Mycolic acids are very long, waxy fatty molecules in the mycobacterial wall. Their ability to keep the red dye carbolfuchsin after acid-alcohol defines acid-fast staining (Ziehl-Neelsen), which is a different stain from the Gram stain. Your answer
- D. Whether the cell wall contains an outer membrane that prevents the primary stain from entering: The outer membrane does not block the primary stain; both Gram-positive and Gram-negative cells take up crystal violet initially. If this option were right, Gram-negative cells would not take the primary stain at all, but the difference is retention after decolourisation. Your answer
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More questions from this set. Start the quiz to answer them and see each explanation.
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1
Which statement correctly separates the two developmental forms of Chlamydia?
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2
A carefully prepared Gram film shows no bacteria. Which interpretation best guides the choice of the next test?
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