The cell wall

The cell wall

Beneath such external structures as capsules, sheaths, and flagella and external to the cytoplasmic membrane is the cell wall, a very rigid structure that gives shape to the cell. Its main function is to prevent the cell from expanding and the eventually bursting because of uptake of water since most bacteria live in hypotonic environments (i.e., environments having the lower osmotic pressure than that exists within the bacterial cells.) The rigidity of the wall can be readily demonstrated by subjecting bacteria to very high pressures or other severe physical conditions: most bacterial cells retain their original shapes during and after such treatments. To obtain isolated cell wall for analysis, bacteria usually must be mechanically disintegrated by drastic means, as by sonic or ultrasonic treatment or by exposure to extremely high pressure with subsequent sudden release of pressure. The broken cell walls are then separated from the rest of the components of the disintegrated cells by differential centrifugation. Isolated cell walls, devoid of other cellular constituents, retain the original contour of the cells from which they were derived.

Among the ordinary or typical bacteria (which are sometimes called eubacteria to distinguish them from the phylogenetically distinct group known as the archaeobacteria.), the walls of Gram-negative species are generally thinner (10-15 nm) than those of Gram-positive species (20-25 nm). The walls of Gram-negative archaeobacteria are also thinner than those of Gram-positive archaeobacteria. Since the chemical composition of the walls of archaeobacteria is quite different from that of eubacteria, wall thickness rather than chemical composition may be the major factor in the gram reaction.

The cell wall constitutes a significant portion of the dry weight of the cell; depending on the species and culture conditions, it may account for as much as 10-40 percent. Bacterial cell walls are usually essential for bacterial growth and division. Cells whose walls have been completely removed (i.e., protoplasts) are incapable of normal growth and division.

Structure and chemical composition (peptidoglycan)

For the eubacteria, the shape-determining part of the cell wall is largely peptidoglycan (sometimes called murein), an insoluble, porous, cross-linked polymer of the enormous strength and rigidity. Peptidoglycan is found only in prokaryotes; it occurs in the form of the “bag-shaped macromolecule” surrounding the cytoplasmic membrane. Peptidoglycan differs somewhat in composition and structure from one species to another, but it is basically a polymer of N-acetylglucosamine, N-acetylmuramic acid, L-alanine, D-glutamate, and a diamine acid (LL- or so-diaminopimelic acid, L-ornithine, or L-aminobutyric acid). The structure of this polymer is depicted. It is important to realize that as tough as peptidoglycan is, it is also in a dynamic state. That is, in order for the cell to grow and divide, portions of the peptidoglycan must continually be degraded by wall-associated hydrolytic enzymes so that new polymer can be added.

Walls of Archaeobacteria

Although most archaeobacteria possess cell walls, these do not contain peptidoglycan, and their cell-wall fine structure and chemical composition are very different from that of eubacteria. Their walls are usually composed of proteins, glycoproteins, or polysaccharides. A few genera, such as Methanobacterium, have walls composed of Pseudomurein, a polymer whose structure superficially resembles eubacterial peptidoglycan but which differs markedly in chemical composition.

Walls of Gram-positive eubacteria

Gram-positive bacteria usually have a much greater amount of peptidoglycan in their cell walls than do gram-negative bacteria; it may account for 50 percent or more of the dry weight of the wall of some gram-positive species, but only about 10 percent of the wall of gram-negative bacteria. Other substances may occur in addition to peptidoglycan. For instances, the walls of streptococcus Pyogenes contain polysaccharides that are covalently linked to the peptidoglycan and which can be extracted with hot dilute hydrochloric acid. The walls of Staphylococcus aureus and Streptococcus feces contain teichoic acids – acidic polymers of ribitol phosphate or glycerol phosphate- which are covalently linked to peptidoglycan and which are extracted with cold dilute acid. Teichoic acids bind magnesium ions, and there is some evidence that they help to protect bacteria from thermal injury by providing an accessible pool of these cations for stabilization of the cytoplasmic membrane. The walls of most gram-positive bacteria contain very little lipid, but those of Mycobacterium, Corynebacterium, and certain other genera are exceptions, being rich in lipids. The ability of mycobacteria to exhibit acid-fast staining (i.e., when stained, the cells cannot be decolorized easily despite treatment with dilute acids) is correlated with the presence of cell wall mycolic acids. A mycolic acid derivative called cord factor (trehalose dimycolate) is toxic and plays an important role in the diseases caused by C. diphtheria and M. tuberculosis.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.