Pili and Capsules

Pili (fimbriae)

Pili, singularly defined as Pilus are internally hollow, non-helical and filamentous appendages. They are thinner, shorter, and more numerous compared to flagella. They do not help for the mobility because of the fact that they are found on non-motile as well as motile species. Pili, however, perform several functions associated with different types of pili. For example, one of the types, known as the F pilus (or sex pilus), functions as the port of entry for genetic material during mating mating of bacteria . Some pili take important part in human infection by helping pathogenic bacteria to attach to epithelial cells lining the respiratory, intestinal, or genitourinary tracts of the humans . This makes the bacteria strongly attached to the host's cell and thus prevents the bacteria from being washed away by the flow of mucous or body fluids and helps the infection to occur.

Capsules

A Covering layer or envelope is formed around the cell wall of some bacteria when they are surrounded by a viscous substance . In the case this layer becomes visualized under light microscopy using special staining methods, it can be termed as a capsule. If the layer is much thin to be seen by light microscopy it is termed as a microcapsule; if it is found in large number so that many cells are embedded in a common matrix, then the the material is called slime.

By the process of light microscopy, capsules appear to be amorphous gelatinous areas surrounding a cell; however, special techniques that are designed to preserve the delicate structures for experimentation by electron microscopy have revealed that capsules contain a mesh or network of fine strands.

In many cases, the capsular material is not highly water-soluble and therefore it does not readily diffuse away from the cells that produce it. In other cases the materials is found to be highly soluble in water and dissolves in the medium, sometimes surprisingly increasing the viscosity of the broth in which the organisms are cultured.

Depending on the bacterial species,.capsules can serve a number of functions. Temporary drying by binding water molecule is prevented by the capsules. Blocking of attachment of bacteriophages is also understood as one of their functions . They inhibit the engulfment of pathogenic bacteria by white blood cells and thus contribute to invasive or infective ability (virulence)i.e. they may be antiphagocytic . They may even help for bacterial attachments to surfaces; for instance, Streptococcus mutants, a bacterium concerned with producing dental caries, can firmly adhere to the smooth surfaces of teeth because of its secretion of capsular glucan insoluble in water. In case the capsules are contained in compounds having an electrical charge, such as sugar-uronic acids, they may promote the stability of bacterial suspension by stopping the cells from aggregating and settling out, because cells having similarly charged surfaces tend to repel one another.

Most bacterial capsules are contained of polysaccharides. Capsules that are composed of only one type of sugar are termed homopolysaccharides; are usually manufactured outside the cell from disaccharides by exocellular enzymes. The synthesis of glucan (a polymer of glucose) from sucrose by S. mutants is an example of it. Other capsules are also composed of several kinds of sugars and are known as heteropolysaccharides; these are usually synthesized with the help of sugars precursors that are activated (energized) within the cell, attached to a lipid carrier molecule, transported across the cytoplasmic membrane, and polymerized outside the cell. The capsule of Klebsiella pneumonia is an example of it.

Few number of capsules are polypeptides. For example, the capsule of the anthrax organism, B. Anthracis, is entirely composed of a polymer of glutamic acid. However, this peptide is found to be unusual one because the glutamic acid is the rare optical isomer rather than the usual L-isomer that is commonly found in nature.

Sheaths

Some species of bacteria, particularly the one from freshwater and marine environments, from chains or trichomes that are enclosed by a hollow, cylindrical tube called sheaths. This structure is most clearly viewed when some of the cells have migrated from it. The sheaths may sometimes become impregnated by the action of ferric or manganese hydroxides, which further strengthen them.

Prosthecae and stalks

Prosthecae, singularly known as Prosthetics, are semi-rigid extensions developed on the cell wall and cytoplasmic membrane and have a diameter always less than that of the cell. They are characteristic of a number of aerobic bacteria especially found in freshwater and marine environments. Some bacterial genera like Caulobacter harbour a single Prostheca; others such as Stella and Ancalomicrobium could have several. Prosthecaes aids for nutrient absorption by increasing surface area of cell , which is advantageous in case of dilute environments. Some Prosthecae bacteria may form a new cell (bud) at the end of a Prosthecae; others could contain an adhesive substance at the end of a Prostheca that aids in attachment to surfaces.

Although the term stalk can sometimes be used interchangeably with the terms Prosthetics or hypha, it is considered better to restrict its use to certain nonliving ribbonlike or tubular appendages that are excreted by the cell, such as those found in the Genera Gallionella or Planctomyces. These stalks aid in attachment of the cells to surfaces.

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.