The cell envelope comprises the inner cell membrane and the cell wall of a bacterium, if present, plus a bacterial outer membrane, if one is present (i.e. in gram-negative bacteria).[1]
Most bacterial cell envelopes fall into two major categories: a gram-positive type and a gram-negative type. These are the envelopes typically found in bacteria staining during a Gram stain. Regardless of its Gram status, a bacterium might have an exterior polysaccharide-containing capsule for further protection of the cell envelope, and is known as a polysaccharide encapsulated bacterium.