The cell envelope comprises the inner

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

and the

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

of a

bacterium Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the ...

. In

Gram-negative bacteria Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...

an outer membrane is also included. This envelope is not present in the

Mollicutes Mollicutes is a class of bacteria distinguished by the absence of a cell wall and its peptidoglycan. The word "Mollicutes" is derived from the Latin ' (meaning "soft" or "pliable"), and ' (meaning "skin"). Individuals are very small, typically on ...

where the cell wall is absent. Bacterial cell envelopes fall into two major categories: a

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...

type which stains purple during

Gram staining Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection. The name comes ...

and a Gram-negative type which stains pink during Gram staining. Either type may have an enclosing capsule of

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

for extra protection. As a group these are known as polysaccharide encapsulated bacteria.

Function

As in other organisms, the bacterial

provides structural integrity to the cell. In

prokaryotes A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...

, the primary function of the cell wall is to protect the cell from internal

turgor pressure Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall. It is also called ''hydrostatic pressure'', and is defined as the pressure in a fluid measured at a certain point within itself when at equilibri ...

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 Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...

(poly-''N''-acetylglucosamine and ''N''-acetylmuramic acid), which is located immediately outside of the

cytoplasmic membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of a Cell (biology), cell from the extrac ...

Peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...

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 ''Mycoplasma'' is a genus of bacteria that, like the other members of the class ''Mollicutes'', lack a cell wall, and its peptidoglycan, around their cell membrane. The absence of peptidoglycan makes them naturally resistant to antibiotics ...

'') 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

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

procedure. It is found exclusively in organisms belonging to the

Actinomycetota The Actinomycetota (or Actinobacteria) are a diverse phylum of Gram-positive bacteria with high GC content. They can be terrestrial or aquatic. They are of great importance to land flora because of their contributions to soil systems. In soil t ...

(or high %G+C Gram-positive organisms) and the

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have Gram-positive cell wall structure. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus). A few Bacillota, such as '' Megasphaera'', ...

(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. Imbedded in the Gram-positive cell wall are polyalcohols called

teichoic acid Teichoic acids (''cf.'' Greek τεῖχος, ''teīkhos'', "wall", to be specific a fortification wall, as opposed to τοῖχος, ''toīkhos'', a regular wall) are bacterial copolymers of glycerol phosphate or ribitol phosphate and carbohyd ...

s, some of which are lipid-linked to form

lipoteichoic acid Lipoteichoic acid (LTA) is a major constituent of the cell wall of gram-positive bacteria. These organisms have an inner (or cytoplasmic) membrane and, external to it, a thick (up to 80 nanometer) peptidoglycan layer. The structure of LTA varies b ...

s. Because lipoteichoic acids are covalently linked to lipids within the

they are responsible for linking the

to the

. Teichoic acids give the gram-positive cell wall an overall negative charge due to the presence of

phosphodiester bonds In chemistry, a phosphodiester bond occurs when exactly two of the hydroxyl groups () in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds. The "bond" involves this linkage . Discussion of phosphodiesters is d ...

between teichoic acid monomers. Outside the cell wall, many Gram-positive bacteria have an

S-layer An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. The S-layers of both archaea and bacteria consists of a Monolayer, monomolecular layer composed of only one (or, in a few c ...

of "tiled" proteins. The S-layer assists attachment and biofilm formation. Outside the S-layer, there is often a capsule of polysaccharides. The capsule helps the bacterium evade host

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

. 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

The

Gram-negative cell wall A bacterium, despite its simplicity, contains a well-developed cell structure which is responsible for some of its unique biological structures and pathogenicity. Many structural features are unique to bacteria, and are not found among archaea or ...

contains a thinner

layer adjacent to the

than the Gram-positive wall, which is responsible for the cell wall's inability to retain the crystal violet stain upon decolourisation with ethanol during

. In addition to the

layer the Gram-negative cell wall also contains an additional outer membrane composed by

phospholipid Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s and

lipopolysaccharide Lipopolysaccharide (LPS), now more commonly known as endotoxin, is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria, such as '' E. coli'' and ''Salmonella'' with a common structural archit ...

s which face into the external environment. The highly charged nature of

s confer an overall negative charge to the Gram -negative cell wall. The chemical structure of the outer membrane

s is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

ic properties of these strains. As a

phospholipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a l ...

, the lipid portion of the outer membrane is largely impermeable to all charged molecules. However, channels called porins are present in the outer membrane that allow for

passive transport Passive transport is a type of membrane transport that does not require energy to move substances across cell membranes. Instead of using cellular energy, like active transport, passive transport relies on the second law of thermodynamics to ...

of many

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

s and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s across the outer membrane. These molecules are therefore present in the

periplasm The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the ''periplasmic space'' in Gram-negative (more accurately "diderm") bacteria. Using cryo-electron micros ...

, the region between the plasma membrane and outer membrane. The

contains the peptidoglycan layer and many proteins responsible for substrate binding or

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

and reception of extracellular signals. The periplasm is thought to exist as a gel-like state rather than a liquid due to the high concentration of proteins and

found within it. Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the

using transport and signaling proteins imbedded there. In nature, many uncultivated Gram-negative bacteria also have an

and a capsule. These structures are often lost during laboratory cultivation.

Mycobacteria

The ''

Mycobacteria ''Mycobacterium'' is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis ('' M. tuberculosis'') a ...

'' (

acid-fast Acid-fastness is a physical property of certain bacterial and eukaryotic cells, as well as some sub-cellular structures, specifically their resistance to decolorization by acids during laboratory staining procedures. Once stained as part of a sa ...

bacteria) have a cell envelope which is not typical of Gram-positives or Gram-negatives. The mycobacterial cell envelope does not consist of the outer membrane characteristic of Gram-negatives, but has a significant peptidoglycan-arabinogalactan-mycolic acid wall structure which provides an external permeability barrier. Therefore, there is thought to be a distinct 'pseudoperiplasm' compartment between the cytoplasmic membrane and this outer barrier. The nature of this compartment is not well understood. Acid-fast bacteria, like ''

'', are resistant to decolorization by acids during staining procedures. The high

mycolic acid Mycolic acids are long fatty acids found in the cell walls of Mycobacteriales taxon, a group of bacteria that includes ''Mycobacterium tuberculosis'', the causative agent of the disease tuberculosis. They form the major component of the cell wall ...

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 The Ziehl-Neelsen stain, also known as the acid-fast stain, is a bacteriological staining technique used in cytopathology and microbiology to identify acid-fast bacteria under microscopy, particularly members of the ''Mycobacterium'' genus. This ...

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 The Chlamydiaceae are a family of gram-negative bacteria that belongs to the phylum Chlamydiota, order Chlamydiales. Chlamydiaceae species express the family-specific lipopolysaccharide epitope αKdo-(2→8)-αKdo-(2→4)-αKdo (previously ca ...

are unique in their morphology as they do not contain detectable amounts of peptidoglycan in the cell wall of their infectious forms. 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.Hatch TP. Disulfide cross-linked envelope proteins: the functional equivalent of peptidoglycan in chlamydiae? J. Bacteriol. 1996 178:1–5. 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. The main

pathogenic bacteria Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and many are Probiotic, beneficial but others can cause infectious diseases. The nu ...

in this class are

mycoplasma ''Mycoplasma'' is a genus of bacteria that, like the other members of the class ''Mollicutes'', lack a cell wall, and its peptidoglycan, around their cell membrane. The absence of peptidoglycan makes them naturally resistant to antibiotics ...

and

ureaplasma ''Ureaplasma'' is a genus of bacteria belonging to the family Mycoplasmataceae. As the name implies, ''Ureaplasma'' is urease positive. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in No ...

L-form bacteria L-form bacteria, also known as L-phase bacteria, L-phase variants or cell wall-deficient bacteria (CWDB), are growth forms derived from different bacteria. They lack cell walls. Two types of L-forms are distinguished: ''unstable L-forms'', spher ...

are strains bacteria that lack cell walls derived from bacteria that normally possess cell walls.

References

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