Gram negative and gram positive bacteria

The difference lies in the cell wall of the two types; in contrast to most Gram-positive bacteria, Gram-negative bacteria have only a few layers of peptidoglycan and a secondary cell membrane made primarily of lipopolysaccharide. The space between the layers of peptidoglican and the secondary cell membrane is called periplasmatic space.

The peptidoglycan layer is thicker in Gram-positive bacteria (20 to 80 nm) than in Gram-negative bacteria (7 to 8 nm). It forms around 90% of the dry weight of gram positive bacteria but only 10% of that of gram negative bacteria.

Gram-positive bacteria are those that are stained dark blue or violet by gram staining, in contrast to gram-negative bacteria, which are not affected by the stain. The stain is caused by a high amount of peptidoglycan in the cell wall, which typically, but not always lacks the secondary membrane and lipopolysaccharide layer found in Gram-negative bacteria.