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Microbiology: Capsule and Gram Staining Techniques

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이 집합의 용어 (21)

  • What is a capsule in bacteria?
    A capsule is a gelatinous outer layer surrounding some bacterial cells that protects against desiccation, phagocytosis, and helps in adherence to surfaces.
  • What advantages does a capsule confer to a bacterial cell?
    Capsules provide protection from immune cells, prevent drying out, and aid in attachment to surfaces or host tissues.
  • What do you stain to observe a bacterial capsule?
    You stain the background and bacterial cells, leaving the capsule unstained and visible as a clear halo around the cell.
  • What are the main steps of the capsule staining procedure?
    1. Apply a negative stain (e.g., India ink or nigrosin) to stain the background. 2. Mix bacteria with skim milk powder. 3. Air dry without heat fixing. 4. Counterstain cells with a simple stain (e.g., crystal violet).
  • Why is skim milk powder used in capsule staining?
    Skim milk powder helps preserve the capsule structure and provides a medium that enhances capsule visibility.
  • What happens if you heat-fix the smear before capsule staining?
    Heat fixing destroys the capsule, causing it to shrink or disappear, making it impossible to observe.
  • Without heat fixing, how do bacterial cells adhere to the slide during capsule staining?
    Cells adhere by air drying and the sticky nature of the smear combined with the skim milk powder.
  • In capsule staining, what is stained: background, cells, or both?
    Both the background and bacterial cells are stained, while the capsule remains unstained, appearing as a clear halo.
  • What type of staining method is Gram staining?
    Gram staining is a differential stain that distinguishes bacteria based on cell wall properties.
  • What is a differential stain and how does it differ from a simple stain?
    A differential stain uses multiple dyes to distinguish cell types or structures, unlike a simple stain which uses one dye to color all cells uniformly.
  • What cellular structure does Gram staining differentiate?
    Gram staining differentiates bacteria based on differences in their cell wall composition, especially peptidoglycan thickness.
  • What color do Gram-positive bacteria take up after Gram staining?
    Gram-positive bacteria appear purple because they retain the crystal violet-iodine complex.
  • What color do Gram-negative bacteria take up after Gram staining?
    Gram-negative bacteria appear pink or red after counterstaining with safranin.
  • Describe the color changes in Gram-positive bacteria during Gram staining steps.
    Gram-positive cells stain purple after crystal violet, remain purple after iodine mordant, stay purple after decolorization, and remain purple after counterstain.
  • Describe the color changes in Gram-negative bacteria during Gram staining steps.
    Gram-negative cells stain purple after crystal violet and iodine, lose color after decolorization, and take up pink/red from the counterstain.
  • Why do Gram-positive and Gram-negative bacteria stain differently?
    Because Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet, while Gram-negative bacteria have a thin peptidoglycan layer and outer membrane that lose the dye during decolorization.
  • What are the correct order and roles of reagents in Gram staining?
    1. Primary stain: Crystal violet. 2. Mordant: Iodine (fixes dye). 3. Decolorizer: Alcohol or acetone (removes dye from Gram-negative). 4. Counterstain: Safranin (stains decolorized cells).
  • What happens if you miss the mordant step in Gram staining?
    Without iodine mordant, the crystal violet is not fixed, causing both Gram-positive and Gram-negative cells to appear pink or colorless.
  • What happens if you decolorize for too long in Gram staining?
    Over-decolorization removes crystal violet from Gram-positive cells, causing false Gram-negative (pink) results.
  • What are common reasons for false Gram stain results?
    Causes include over-decolorization, old cultures, improper smear thickness, and skipping steps like mordant or counterstain.
  • How can you identify Gram-positive and Gram-negative cells in a microscopic slide?
    Gram-positive cells appear purple and Gram-negative cells appear pink/red after completing the Gram stain procedure.