Unlike Gram-positive bacteria, which asuume a violet color in Gram staining, Gram negative bacteria incorporate the counterstain rather than the primary stain. Because the cell wall of Gram(-) bacteria is high in lipid content and low in peptidoglycan content, the primary crystal-violet escapes from the cell when the decolorizer is added. This is because primary stains like to bind with peptidoglycan- something the G(-) cell has very little of. Gram(-) bacteria cause a lot of problems because many species are pathogenic. This pathogenic capability is usually associated with certain components of their cell walls, particularly the lipopolysaccharide (endotoxin) layer. Remember the Black Plague which wiped out a a third of the population of Europe? It was caused by the tiny G(-) rod, Yersinia pestis. Most enteric (bowel related) illnesses can also be attributed to this group of bacteria. Some of my favorites are Salmonella food poisoning. If you choose, you can directly access the diseases section of this tutorial to learn more.
