the simple stains:
Because most cells are transparent, staining them with dyes makes them easier to see and discern. Cells are stained with a colored dye that makes them more visible under the light microscope. You can then determine cell morphology, cell size, and cell arrangement. Stains are solutions. They consist of a solvent (the liquid, usually water or ethanol alcohol), as well as a solute, the colored molecule (the chromogen, which may be a benzene derivative). The chromophore part of the chromogen is what gives it its color. There may be more than one. Dyes contain charges that enable them to bind with the cell membrane or cell wall of the cell. These are called auxochromes, which may be either positively or negatively charged. Basic stains contain auxochromes that become positively charged when they either gain a hydrogen ion or lose a hydroxide ion, which causes them to become attracted to the negative charges on the surface of the bacterial cells. This enables the cell to retain the dye or color.
Basic simple stains include the following:
Negative stains, on the other hand, are acidic, rather than basic stains, containing a negatively charged chromogen, which is repelled by the like charges (negative) on the cells. This means that the background is colored, but the cells remain transparent. Common acidic stains used include India Ink, Nigrosin, or Congo Red.
Basic simple stains include the following:
- Methylene blue
- Crystal violet
- Safranin
- Negative stain (Nigrosin or India Ink)
- Congo Red
Negative stains, on the other hand, are acidic, rather than basic stains, containing a negatively charged chromogen, which is repelled by the like charges (negative) on the cells. This means that the background is colored, but the cells remain transparent. Common acidic stains used include India Ink, Nigrosin, or Congo Red.
heat fixing:
Before the stain will stick to the cells, bacterial smears have to be made in order to "fix" the cells to the slide first. This is referred to as "heat fixation". Heat fixing kills the bacteria, makes them "stick" to the slide, and make the cytoplasmic proteins coagulate so you can see them better when stained. The cells also become distorted to some extent.