Gram-negative bacteria are those bacteria that do not retain crystal violet dye in the Gram staining protocol.^[1] 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. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their cell walls.On the other hand, Gram-positive bacteria will retain the crystal violet dye when washed in a decolorizing solution.

Many species of Gram-negative bacteria are pathogenic, meaning that 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.^[1] In humans, LPS triggers an innate immune response characterized by cytokine production and immune system activation. Inflammation is a common result of cytokine (from the greek cyto=cell, kinesis=movement) production, which can also produce host toxicity.

Photo: Gram-negative Pseudomonas aeruginosa bacteria (pink-red rods)