Stains increase the contrast and allow the components of the cell to become more visible.The specimen is placed on a slide and allowed to dry. It can be heat fixed by passing through a flame. The stain is then applied with a dropper pipette and a cover slip is placed over the specimen. The excess stain is removed using a paper towel.Differential staining can distinguish between types of organisms or stain different parts of the cell.
- Crystal violet or methylene blue are positively charged and attracted to negatively charged particles in the cytosol and organelles.
- Congo red and nigrosin are negative and stain the outside of cells
- Gram stain (crystal violet, then iodine followed by washing with alcohol) is retained by gram positive bacteria with thicker cell walls. Gram negative bacteria with thinner cell walls appear red when counterstained with safranin dye
- Acid-fast technique involves using a lipid solvent to carry carbolfuchsin dye into bacterial cells. The cells are then washed with dilute acid-alcohol solution. Mycobacterium retain the bright red stain.