Cell Walls And Germ Stain Machanism Flashcards
(17 cards)
What is the basis of the Gram stain mechanism?
A: Differences in cell wall structure between gram-positive and gram-negative bacteria and their reactions to various reagents
What is the primary stain used in the Gram stain procedure, and what color does it initially stain both types of cells?
A: Crystal violet, which stains both gram-positive and gram-negative cells purple
What role does iodine play in the Gram stain process?
A: It acts as a mordant, forming large, water-insoluble crystals with crystal violet.
What happens to gram-negative cells when alcohol is applied during the Gram stain?
A: The alcohol dissolves their outer membrane, allowing the crystal violet-iodine complex to wash out, leaving them colorless.
Why do gram-negative cells appear pink or red after the Gram stain process?
A: They are colorless after the alcohol wash, so they absorb safranin, the counterstain, which turns them pink or red.
Why don’t gram-positive cells appear pink or red after safranin is applied in the Gram stain process?
A: The darker purple dye from crystal violet masks the pink or red color of safranin in gram-positive cells.
Why might some gram-positive cells show a gram-negative response?
Dead cells or aged cultures of certain gram-positive genera may lose their ability to retain the crystal violet stain.
Which bacteria have no cell walls and can pass through most bacterial filters, initially being mistaken for viruses?
.
A: Mycoplasmas
What is unique about the plasma membranes of Mycoplasma bacteria?
A: They contain sterols, which help protect against lysis.
What is pseudomurein, and which organisms have cell walls containing it?
A: Pseudomurein is a substance similar to peptidoglycan found in Archaea cell walls, containing N-acetyltalosaminuronic acid instead of NAM.
Why do Archaea generally appear gram-negative when Gram-stained?
A: They lack peptidoglycan, which is essential for Gram stain retention.
What is the purpose of the acid-fast stain?
A: To identify bacteria of the genus Mycobacterium and pathogenic species of Nocardia
What component in acid-fast bacteria prevents the uptake of dyes, including the Gram stain?
A: Mycolic acid, a hydrophobic waxy lipid present in high concentrations in the cell wall.
Where is mycolic acid located in the cell wall of acid-fast bacteria?
A: Outside a thin layer of peptidoglycan, held together by a polysaccharide.
What effect does the hydrophobic waxy cell wall have on Mycobacterium cultures?
A: It causes broth cultures to clump and stick to the walls of the flask.
Why do acid-fast bacteria retain the red color of carbolfuchsin after washing with acid-alcohol?
A: Because carbolfuchsin is more soluble in mycolic acid than in acid-alcohol.
What happens to acid-fast bacteria if their mycolic acid layer is removed?
A: They will stain gram-positive with the Gram stain.
