gram-negative bacteria

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. 

In a Gram stain test, a counterstain (commonly safranin) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color. This is because of the existence of an outer membrane preventing the penetration of the stain. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their bacterial cell walls. Gram-positive bacteria will retain the crystal violet dye when washed in a decolorizing solution. Compared with gram-positive bacteria, gram-negative are more resistant against antibodies, because of their impenetrable wall.

The pathogenic capability of Gram-negative bacteria is often associated with certain components of Gram-negative cell envelope, in particular, the lipopolysaccharide layer.