Cell envelope

Source: Wikipedia, the free encyclopedia.

The cell envelope comprises the inner

bacterium. In gram-negative bacteria an outer membrane is also included.[1] This envelope is not present in the Mollicutes
where the cell wall is absent.

Bacterial cell envelopes fall into two major categories: a

polysaccharide encapsulated bacteria
.

Function

As in other organisms, the bacterial

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.

Types

The gram-positive cell wall

gram-positive cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid; Teichoic acids
not shown.

The

phosphodiester bonds
between teichoic acid monomers.

Outside the cell wall, many gram-positive bacteria have an

capsule of polysaccharides. The capsule helps the bacterium evade host phagocytosis
. In laboratory culture, the S-layer and capsule are often lost by reductive evolution (the loss of a trait in absence of positive selection).

The gram-negative cell wall

gram-negative cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid and the outer membrane containing lipopolysaccharide
.

The

Gram staining. In addition to the peptidoglycan layer the gram-negative cell wall also contains an additional outer membrane composed by phospholipids and lipopolysaccharides which face into the external environment. The highly charged nature of lipopolysaccharides confer an overall negative charge to the gram -negative cell wall. The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigenic
properties of these strains.

As a

cytoplasmic membrane
using transport and signaling proteins imbedded there.

In nature, many uncultivated gram-negative bacteria also have an

capsule
. These structures are often lost during laboratory cultivation.

Mycobacteria

The

Mycobacteria, are resistant to decolorization by acids during staining procedures. The high mycolic acid content of Mycobacteria, is responsible for the staining pattern of poor absorption followed by high retention. The most common staining technique used to identify acid-fast bacteria is the Ziehl–Neelsen stain
or acid-fast stain, in which the acid fast bacilli are stained bright red and stand out clearly against a blue background.

Bacteria lacking a peptidoglycan cell wall

The obligate intracellular bacteria in the family Chlamydiaceae are unique in their morphology as they do not contain detectable amounts of peptidoglycan in the cell wall of their infectious forms.[3] Instead, the extracellular forms of these gram-negative bacteria maintain their structural integrity by relying on a layer of disulfide bond cross-linked cysteine-rich proteins, which is located between cytoplasmic membrane and outer membrane in a manner analogous to the peptidoglycan layer in other gram-negative bacteria.[4] In the intracellular forms of the bacterium the disulfide cross linkage is not found, which confers this form more mechanically fragile.

The cell envelopes of the bacterial class of

mollicutes do not have a cell wall.[5] The main pathogenic bacteria in this class are mycoplasma and ureaplasma.[5]

L-form bacteria are strains bacteria that lack cell walls derived from bacteria that normally possess cell walls.[6]

See also

References

  1. ^ "envelope" at Dorland's Medical Dictionary
  2. ^ IC Sutcliffe, DJ Harrington.Lipoproteins of mycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components. FEMS Microbiology Reviews 28 (2004) 645-759
  3. ^ Chopra I, Storey C, Falla TJ, Pearce JH. Antibiotics, peptidoglycan synthesis and genomics: the chlamydial anomaly revisited. Microbiology. 1998 144 ( Pt 10):2673-8.
  4. ^ Hatch TP. Disulfide cross-linked envelope proteins: the functional equivalent of peptidoglycan in chlamydiae? J. Bacteriol. 1996 178:1–5.
  5. ^
    PMID 12663864
    .
  6. PMID 19212404
    .
Retrieved from "https://en.wikipedia.org/w/index.php?title=Cell_envelope&oldid=1215153834"