The cell wall is the layer that lies just outside the plasma membrane. It is one of the most important structures for several reasons: it helps maintain cell shape and protect the cell from osmotic lysis; it can protect the cell from toxic substances; and in pathogens, it can contribute to pathogenicity.
Cell walls are so important that most bacteria have them. Those that do not have other features that fulfill cell wall function. Bacterial cell wall synthesis is targeted by several antibiotics. Therefore, it is important to understand cell wall structure.
Overview of Bacterial Cell Wall Structure
After Christian Gram developed the Gram stain in 1884, it soon became evident that most bacteria could be divided into two major groups based on their response to the Gram-staining procedure. Gram-positive bacteria stained purple, whereas Gram-negative bacteria were pink or red.
The true structural difference between these two groups did not become clear until the advent of the transmission electron microscope. Here we describe the long-held models of Gram-positive and Gram-negative cell walls developed from these studies.
More recent studies of diverse groups of bacteria have shown that these models do not hold true for all bacteria. Because of the ongoing discussions related to these new studies, we will refer to bacteria that fit the models as being typical Gram-positive or typical Gram-negative bacteria.
The cell walls of Bacillus subtilis and many other typical Gram-positive bacteria consist of a single, 20- to 80-nm-thick homogeneous layer of peptidoglycan (murein) lying outside the plasma membrane. In contrast, the cell walls of E. coli and many other typical Gram-negative bacteria have two distinct layers: a 2- to 7-nm-thick peptidoglycan layer covered by a 7- to 8-nm-thick outer membrane.
One important feature seen in typical Gram-negative bacteria is a space between the plasma membrane and the outer membrane. It also is sometimes observed between the plasma membrane and cell wall in typical Gram-positive bacteria. This space is called the periplasmic space. The substance that occupies the periplasmic space is the periplasm.
Cell wall of Gram-Positive Bacteria
Most bacteria that stain Gram positive belong to the phyla Firmicutes and Actinobacteria, and most of these bacteria have thick cell walls composed of peptidoglycan and large amounts of other polymers such as teichoic acids.
Teichoic acids are polymers of glycerol or ribitol joined by phosphate groups. Some teichoic acids are covalently linked to peptidoglycan and are referred to as wall teichoic acids. Others are covalently connected to the plasma membrane; they are called lipoteichoic acids.
Wall teichoic acids extend beyond the surface of the peptidoglycan. They are negatively charged and help give the cell wall its negative charge. Teichoic acids are not present in other bacteria.
Teichoic acids have several important functions. They help create and maintain the structure of the cell envelope by anchoring the wall to the plasma membrane. They are important during cell division, and they protect the cell from harmful substances in the environment (e.g., antibiotics and host defense molecules). In addition, they function in ion uptake and are involved in binding pathogenic species to host tissues, thus initiating the infectious disease process.
The periplasmic space lies between the plasma membrane and the cell wall and is so narrow that it is often not visible. The periplasm has relatively few proteins; this is probably because the peptidoglycan sacculus is porous and many proteins translocated across the plasma membrane pass through the sacculus.
Some secreted proteins are enzymes called exoenzymes. Exoenzymes often serve to degrade polymers such as proteins and polysaccharides that would otherwise be too large for transport across the plasma membrane; the degradation products, the monomer building blocks, are then taken up by the cell. Those proteins that remain in the periplasmic space are usually attached to the plasma membrane.
In addition to the polymers embedded in the peptidoglycan sacculus, there are often proteins associated with its surface. These proteins are involved in interactions of the cell with its environment. Some are noncovalently bound to teichoic acids or other cell wall polymers. Other surface proteins are covalently attached to the peptidoglycan.
Membrane-bound enzymes called sortases catalyze the formation of covalent bonds that join these proteins to the peptidoglycan. Many covalently attached proteins have roles in virulence. For example, the M protein of pathogenic streptococci aids in adhesion to host tissues and interferes with host defenses.
Cell wall of Gram- Negative Bacteria
As just noted, most bacteria that stain Gram positive belong to the phyla Firmicutes and Actinobacteria. With a few exceptions’ bacteria belonging to the remaining phyla stain Gram negative. Even a brief inspection.
The typical Gram-negative cell walls are more complex than typical Gram-positive walls. One of the most striking differences is the paucity of peptidoglycan. The peptidoglycan layer is very thin (2 to 7 nm, depending on the bacterium) and sits within the periplasmic space.
The periplasmic space is usually 30 to 70 nm wide. Some studies indicate that it may constitute about 20 to 40% of the total cell volume. Thus it is much larger than that observed in typical Gram-positive cells. When cell walls are disrupted carefully or removed without disturbing the underlying plasma membrane, periplasmic enzymes and other proteins are released and may be easily studied.
Some periplasmic proteins participate in nutrient acquisition—for example, hydrolytic enzymes and transport proteins. Some periplasmic proteins are involved in energy conservation. For instance, some bacteria have electron transport proteins in their periplasm (e.g., denitrifying bacteria, which convert nitrate to nitrogen gas). Other periplasmic proteins are involved in peptidoglycan synthesis and modification of toxic compounds that could harm the cell.
The outer membrane lies outside the thin peptidoglycan layer. It is linked to the cell by Braun’s lipoprotein, the most abundant protein in the outer membrane. This small lipoprotein is covalently joined to the underlying peptidoglycan and is embedded in the outer membrane by its hydrophobic end.
Possibly the most unusual constituents of the outer membrane are its lipopolysaccharides (LPSs). These large, complex molecules contain both lipid and carbohydrate, and consist of three parts: (1) lipid A, (2) the core polysaccharide, and (3) the O side chain.
The LPS from Salmonella spp. has been studied most, and its general structure is described here. Lipid A contains two glucosamine sugar derivatives, each with fatty acids and phosphate attached. The fatty acids of lipid A are embedded in the outer membrane, while the remainder of the LPS molecule projects from the surface.
The core polysaccharide is joined to lipid A and is constructed of 10 sugars, many of them unusual in structure. The O side chain or O antigen is a polysaccharide chain extending outward from the core. It has several peculiar sugars and varies in composition between bacterial strains.
LPS has many important functions.
(1) It contributes to the negative charge on the bacterial surface because the core polysaccharide usually contains charged sugars and phosphate
(2) It helps stabilize outer membrane structure because lipid A is a major constituent of the exterior leaflet of the outer membrane.
(3) It helps create a permeability barrier. The geometry of LPS and interactions between neighboring LPS molecules are thought to restrict the entry of bile salts, antibiotics, detergents, and other toxic substances that might kill or injure the bacterium.
(4) LPS helps protect pathogenic bacteria from host defenses. The O side chain of LPS is also called the O antigen because it elicits an immune response by an infected host. This response involves the production of antibodies that bind the strain-specific form of LPS that elicited the response.
For example, microbiologists refer to specific strains of Gram-negative bacteria using the O antigen, such as E. coli O157; here the O side chain is the antigenic type number 157. Unfortunately, many bacteria can rapidly change the antigenic nature of their O side chains, thus thwarting host defenses.
(5) Importantly, the lipid A portion of LPS can act as a toxin and is called endotoxin; it causes some of the symptoms that arise in infections by Gram negative pathogens. If LPS or lipid A enters the bloodstream, a form of septic shock develops for which there is no direct treatment.
As you have just seen, the makeup of the outer membrane differs from that of the plasma membrane. The two membranes also differ in terms of permeability. Even though LPS helps create a permeability barrier, the outer membrane is more permeable than the plasma membrane and permits the passage of small molecules such as glucose and other monosaccharides.
This greater permeability is due to the presence of porin proteins. Most porin proteins cluster together to form a trimer in the outer membrane. Each porin protein spans the outer membrane and is more or less tube-shaped; its narrow, water-filled channel allows passage of molecules smaller than about 600 daltons.
However, larger molecules such as vitamin B12 cross the outer membrane through the action of specific outer membrane carrier proteins that deliver the molecule to ABC transporters in the plasma membrane.