Gram-negative bacteria are those that do not retain crystal violet dye in the Gram staining protocol. Gram-positive bacteria will retain the dark blue dye after an alcohol wash. In a Gram stain test, a counterstain (commonly Safranin) is added after the crystal violet, coloring all Gram-negative bacteria a red or pink color. The test itself is useful in classifying two distinctly different types of bacteria based on structural differences in their cell walls.
Many species of Gram-negative bacteria are pathogenic, meaning they can cause disease in a host organism. This pathogenic capability is usually associated with certain components of Gram-negative cell walls, in particular the lipopolysaccharide (also known as LPS or endotoxin) layer. The LPS is the trigger which the body's innate immune response receptors sense to begin a cytokine reaction. It is toxic to the host. It is this response which begins the inflammation cycle in tissues and blood vessels.