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Concept
Cell envelope
Summary
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 and a gram-negative type, distinguished by Gram staining. Either type may have an enclosing capsule of polysaccharides for extra protection. As a group these are known as polysaccharide encapsulated bacteria.
As in other organisms, the bacterial cell wall provides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan (poly-N-acetylglucosamine and N-acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g. intracellular parasites such as Mycoplasma) contain peptidoglycan, not all cell walls have the same overall structures. This is notably expressed through the classification into gram positive and gram negative bacteria.
The gram-positive cell wall is characterized by the presence of a very thick peptidoglycan layer, which is responsible for the retention of the crystal violet dyes during the Gram staining procedure. It is found exclusively in organisms belonging to the Actinomycetota (or high %G+C gram-positive organisms) and the Bacillota (or low %G+C gram-positive organisms). Bacteria within the Deinococcota group may also exhibit gram-positive staining behavior but contain some cell wall structures typical of gram-negative organisms.
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