Cell envelope

The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In gram-negative bacteria an outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is absent.

Bacterial cell envelopes fall into two major categories: a gram-positive type which stains purple during Gram staining and a gram-negative type which stains pink during gram staining. Either type may have an enclosing capsule of polysaccharides for extra protection. As a group these are known as polysaccharide encapsulated bacteria.