Bacterial Cell Envelopes and What the Gram Stain Reveals · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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What the Gram stain reveals

Science · Microbiology · ages 18-19
Name ______________________   Date ____________

  1. Why do typical Gram-positive cells retain purple dye during correct decolorization?

    • Safranin makes complexes that stay in the wall
    • Thick peptidoglycan retains the dye complexes
    • The outer membrane keeps dye inside the cell

  2. What is the order of the four Gram stain reagents?

    • Iodine, safranin, crystal violet, decolorizer
    • Decolorizer, crystal violet, safranin, iodine
    • Crystal violet, iodine, decolorizer, safranin

  3. During correct decolorization, thick peptidoglycan helps Gram-positive cells retain dye complexes, while thin peptidoglycan fails to retain them after the Gram-negative outer membrane is disrupted.

    Circle one:   True   False

  4. Which layer is absent from the usual Gram-positive envelope but can restrict antibiotic entry in Gram-negative cells?

    • The inner membrane
    • The peptidoglycan wall
    • The outer membrane

  5. A fresh Gram-positive culture receives too much decolorizer and then safranin. Which misleading result can occur?

    • Some cells lose both dyes and remain colourless
    • Some cells lose purple dye and then stain pink
    • Some cells retain more purple dye after the wash

  6. A susceptible bacterium is growing in dilute surroundings. A beta-lactam blocks new peptidoglycan cross-links. Which outcome can follow?

    • The cell may shrink as water moves out of its cytoplasm
    • The cell may burst as water enters and internal pressure rises
    • The cell wall may strengthen as fewer cross-links are made

  7. Two bacterial isolates both stain pink. What is needed to assess whether a particular antibiotic inhibits each isolate?

    • Compare each isolate's response in susceptibility testing
    • Compare the isolates' shades of pink after counterstaining
    • Compare the isolates' peptidoglycan thickness in their walls

  8. A Gram-negative cell loses purple dye during decolorization. Lee says its outer membrane must let antibiotics enter freely. Which reply explains the problem?

    • Alcohol leaves the outer membrane intact, so purple dye loss measures antibiotic entry.
    • Safranin tests membrane channels, so its colour predicts antibiotic entry.
    • Alcohol disrupts the membrane, so dye loss does not test intact-cell drug entry.
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Answer key

For grown-ups. Fold this page away before handing over the rest.

What the Gram stain reveals W1-mt_Q5zM0oO3Vu-s1

  1. Thick peptidoglycan retains the dye complexes · During correct decolorization, the thick peptidoglycan of typical Gram-positive cells retains crystal violet-iodine complexes.
  2. Crystal violet, iodine, decolorizer, safranin · Crystal violet stains first, iodine forms dye complexes, decolorizer removes dye from some cells, and safranin counterstains.
  3. True · The statement links the two envelope structures to their contrasting dye-retention results during correct decolorization.
  4. The outer membrane · The usual Gram-negative envelope has an outer membrane that restricts some antibiotics. The usual Gram-positive envelope lacks that layer.
  5. Some cells lose purple dye and then stain pink · Excess decolorizer can remove purple dye from Gram-positive cells. Safranin then colours those decolorized cells pink.
  6. The cell may burst as water enters and internal pressure rises · Blocked cross-linking weakens the growing wall. Water entering from dilute surroundings can raise pressure and cause lysis.
  7. Compare each isolate's response in susceptibility testing · Susceptibility testing assesses each isolate's response to the antibiotic. Stain colour or wall thickness alone does not measure that response.
  8. Alcohol disrupts the membrane, so dye loss does not test intact-cell drug entry. · Alcohol changes the outer membrane during staining. Dye loss in that altered sample does not measure antibiotic entry through an intact membrane.
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