Cellular Biology

Extermal Structures Prokaryotic Cells



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Many prokaryotes  have distinct appendages, delicate strands made of protein, that extend beyond the surface of the cell. Whip-like flagella enable some bacteria to move about, shorter extensions, called fimbriae, enable bacteria to adhere to surfaces and, for pathogenic (bad guy) bacteria, allow them to infect the cells of a potential host. Some bacteria even have extensions called sex pili, that enable them to share some of their genes with other bacteria. Here is a breakdown of the main bacterial surface appendages.

* Bacterial Flagella Placement and Number *

Flagella (singular flagellum) are long, thin extensions, like rotating propellers, that allow some bacteria to move about freely in aqueous environments. Some eukaryotic cells (such as human sperm for example) also have flagella, but prokaryotic and eukaryotic flagella are dissimilar, constructed and operating in distinct ways.

The placement and number of bacterial flagella are distinguished using specific terminology. The four main placements of prokaryotic flagella are:

    * Monotrichous: Only one flagellum extending from one end of the bacterium
    * Amphitrichous: One flagellum extending from each end of the bacterium
    * Lophotrichous: Many flagella extending from one end of the bacterium
    * Peritrichous: Falgella extending from many different sites on the bacterium

Bacteria flagella are a wonder in locomotive ability, propelling a bacterium 20 bacterial cell lengths per second.

Many different types of bacteria have flagella, but one interesting example of a flagellated bacterium is Helicobacter pylori, a spiral-shaped bacillus that causes stomach and duodenal ulcers. H. pylori uses its multiple, polar flagella to penetrate the thick coating of the stomach epithelium, burrowing in beyond the reach of caustic stomach acids.

* Spirochete Endoflagela (Axial Filaments) *

Spiral-shaped bacteria, called spirochetes, have special flagella that are tightly wound around their tiny cork-screw shape. These unique flagella, called endoflagella, together form what is called an axial filament. Rotation of the endoflagella causes the axial filament to rotate, and propels the spirochete in a twisting motion. Treponema pallidum, the bacterium that causes syphilis, moves around this way.

* Gram-negative Bacterial Fimbriae *

Most Gram-negative bacteria have these short, fine appendages surrounding the cell; in contrast with Gram+ bacteria, which never have fimbriae.

These protein projections have no role in motility, but instead help bacteria adhere to surfaces and infect host cells. Fimbriae are a major factor in bacterial virulence (the ability of a bacterium to cause disease.

Bacteria, such as Neisseria gonorrhoeae and N. menigitidis, use their fimbriae to attach onto host cells, causing gonorrhoeae and bacterial meningitis respectively. These bacteria would be avirulent (unable to cause disease) without the use of their fimbriae.

* Conjugation Pili of Bacteria *

These proteinacious tubes are longer than fimbriae, and usually shorter than flagella. They are used, like grappling hooks, to move bacteria around. Some pili, called conjugation pili, are used to achieve what is a close to having sex as a bacterium can come. One bacterium may use a conjugation pilus to ‘dock’ with another bacterium, then the donor bacterium transmits some of its genetic information to the recipient, a form of horizontal gene transfer.

* Sources *

Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.

Perry, J. and Stanley, J. (1997) Microbiology: Dynamics and Diversity. Saunders College Publishing.

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