Bacterial cell structure

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 eukaryotes. Because of the simplicity of bacteria relative to larger organisms and the ease with which they can be manipulated experimentally, the cell structure of bacteria has been well studied, revealing many biochemical principles that have been subsequently applied to other organisms.

Cell morphology

Perhaps the most elemental structural property of bacteria is their morphology. Typical examples include:

Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages and some produce elaborate structures bearing reproductive spores. Bacteria generally form distinctive cell morphologies when examined by light microscopy and distinct colony morphologies when grown on Petri plates.
Perhaps the most obvious structural characteristic of bacteria is their small size. For example, Escherichia coli cells, an "average" sized bacterium, are about 2 μm long and 0.5 μm in diameter, with a cell volume of 0.6–0.7 μm³. This corresponds to a wet mass of about 1 picogram, assuming that the cell consists mostly of water. The dry mass of a single cell can be estimated as 23% of the wet mass, amounting to 0.2 pg. About half of the dry mass of a bacterial cell consists of carbon, and also about half of it can be attributed to proteins. Therefore, a typical fully grown 1-liter culture of Escherichia coli yields about 1 g wet cell mass. Small size is extremely important because it allows for a large surface area-to-volume ratio which allows for rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area-to-volume ratios the diffusion of nutrients and waste products across the bacterial cell membrane limits the rate at which microbial metabolism can occur, making the cell less evolutionarily fit. The reason for the existence of large cells is unknown, although it is speculated that the increased cell volume is used primarily for storage of excess nutrients.
Comparison of a typical bacterial cell and a typical human cell :

	Bacterial cell	Human cell	Comparison
Diameter	1μm	10μm	Bacterium is 10 times smaller.
Surface area	3.1μm²	314μm²	Bacterium is 100 times smaller.
Volume	0.52μm³	524μm³	Bacterium is 1000 times smaller.
Surface-to-volume ratio	6	0.6	Bacterium is 10 times greater.

Cell wall

The cell envelope is composed of the cell membrane and the cell wall. 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 which is located immediately outside of the cell membrane. Peptidoglycan is made up of a polysaccharide backbone consisting of alternating N-Acetylmuramic acid and N-acetylglucosamine residues in equal amounts. 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 contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in some eukaryotes, several antibiotics stop bacterial infections by interfering with cell wall synthesis, while having no effects on human cells which have no cell wall, only a cell membrane. There are two main types of bacterial cell walls, those of Gram-positive bacteria and those of Gram-negative bacteria, which are differentiated by their Gram staining characteristics. For both these types of bacteria, particles of approximately 2 nm can pass through the peptidoglycan. If the bacterial cell wall is entirely removed, it is called a protoplast while if it's partially removed, it is called a spheroplast. Beta-lactam antibiotics such as penicillin inhibit the formation of peptidoglycan cross-links in the bacterial cell wall. The enzyme lysozyme, found in human tears, also digests the cell wall of bacteria and is the body's main defense against eye infections.

Gram-positive cell wall

Gram-positive cell walls are thick and the peptidoglycan layer constitutes almost 95% of the cell wall in some Gram-positive bacteria and as little as 5-10% of the cell wall in Gram-negative bacteria. The peptidoglycan layer takes up the crystal violet dye and stains purple in the Gram stain. Bacteria within the Deinococcota group may also exhibit Gram-positive staining but contain some cell wall structures typical of Gram-negative bacteria.
The cell wall of some Gram-positive bacteria can be completely dissolved by lysozymes which attack the bonds between N-acetylmuramic acid and N-acetylglucosamine. In other Gram-positive bacteria, such as Staphylococcus aureus, the walls are resistant to the action of lysozymes. They have O-acetyl groups on carbon-6 of some muramic acid residues.
The matrix substances in the walls of Gram-positive bacteria may be polysaccharides or teichoic acids. The latter are very widespread, but have been found only in Gram-positive bacteria. There are two main types of teichoic acid: ribitol teichoic acids and glycerol teichoic acids. The latter one is more widespread. These acids are polymers of ribitol phosphate and glycerol phosphate, respectively, and only located on the surface of many Gram-positive bacteria. However, the exact function of teichoic acid is debated and not fully understood. Some are lipid-linked to form lipoteichoic acids. Because lipoteichoic acids are covalently linked to lipids within the cytoplasmic membrane they are responsible for linking and anchoring the peptidoglycan to the cytoplasmic membrane. Lipotechoic acid is a major component of the gram-positive cell wall. One of its purposes is providing an antigenic function. The lipid element is to be found in the membrane where its adhesive properties assist in its anchoring to the membrane. Teichoic acids give the gram-positive cell wall an overall negative charge due to the presence of phosphodiester bonds between teichoic acid monomers.
Outside the cell wall, many gram-positive bacteria have an S-layer 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. In laboratory culture, the S-layer and capsule are often lost by reductive evolution.

Gram-negative cell wall

Gram-negative cell walls are much thinner than the Gram-positive cell walls, and they contain a second plasma membrane superficial to their thin peptidoglycan layer, in turn adjacent to the cytoplasmic membrane. Gram-negative bacteria stain as pink in the Gram stain. The chemical structure of the outer membrane's lipopolysaccharide is often unique to specific bacterial sub-species and is responsible for many of the antigenic properties of these strains.
In addition to the peptidoglycan layer the Gram-negative cell wall also contains an additional outer membrane composed of 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, and is responsible for many of their antigenic properties.
As a phospholipid bilayer, 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 of many ions, sugars and amino acids across the outer membrane. These molecules are therefore present in the periplasm, the region between the plasma membrane and outer membrane. The periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysis 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 peptidoglycan 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 cytoplasmic membrane using transport and signaling proteins imbedded there.
Many uncultivated Gram-negative bacteria also have an S-layer and a capsule. These structures are often lost during laboratory cultivation.

Plasma membrane

The plasma membrane or bacterial cytoplasmic membrane is composed of a phospholipid bilayer and thus has all of the general functions of a cell membrane such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules into the cell. In addition to these functions, prokaryotic membranes also function in energy conservation as the location about which a proton motive force is generated. Unlike eukaryotes, bacterial membranes generally do not contain sterols. However, many microbes do contain structurally related compounds called hopanoids which likely fulfill the same function. Unlike eukaryotes, bacteria can have a wide variety of fatty acids within their membranes. Along with typical saturated and unsaturated fatty acids, bacteria can contain fatty acids with additional methyl, hydroxy or even cyclic groups. The relative proportions of these fatty acids can be modulated by the bacterium to maintain the optimum fluidity of the membrane.
Gram-negative and mycobacteria have an inner and outer bacteria membrane. As a phospholipid bilayer, the lipid portion of the bacterial outer membrane is impermeable to charged molecules. However, channels called porins are present in the outer membrane that allow for passive transport of many ions, sugars and amino acids across the outer membrane. These molecules are therefore present in the periplasm, the region between the cytoplasmic and outer membranes. The periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysis and reception of extracellular signals. The periplasm is thought to exist in a gel-like state rather than a liquid due to the high concentration of proteins and peptidoglycan 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 cytoplasmic membrane using transport and signaling proteins imbedded there.