Exercise 5: Differential stains Flashcards
What is a differential stain used for?
To learn more about the cell, such as whether it contains an endospore or possesses a capsule, we must resort to a differential stain.
What does a Gram stain differentiate for?
It distinguishes between two type of cell walls found in bacteria.
What are the steps of Gram stain procedure?
- First dye added to the smear is crystal violet.
- Followed by an iodine solution.
- Next decolorizing with acetone-alcohol.
- Lastly, a safranin counterstain is applied.
What does iodine do when added to the crystal violet dye?
The iodine acts as a mordant (a specialized term used in dyeing), combining with the crystal violet to form an insoluble colored compound in the bacteria being stained. This insoluble precipitate is called the crystal violet-iodine complex.
What do gram positive bacteria look like?
Bacteria that resist decolorizing and retain the crystal violet-iodine complex appear purple and are called gram positive.
What do gram negative bacteria look like?
Conversely, those cells that decolorize or lose the crystal violet-iodine complex more rapidly will accept the safranin counterstain and appear pink/red. These are the gram negative bacteria.
What is acid-fast stain used for?
An acid-fast stain is used for bacteria with a high mycolic acid content in their cell wall, making them resistant to the decolorizing step of the Gram stain.
What is capsule stain used for?
A capsule stain highlights the polysaccharide layer that some bacteria have, using an acidic stain on the background and a basic stain on the cell, leaving the capsule as an unstained halo around the cell.
What is endospore stain used for?
An endospore stain uses a heating step to drive one stain into the endospore coat to allow identification of the bacterial endospores, while using a second stain on the vegetative cells.
