Accessory structures of bacteria

Scourge or Cilia

The flagellum is an organ of locomotion typical of cylindrical-shaped bacteria (bacilli).

Depending on the number and location of these flagella, the bacteria are divided into:



Flagella - whose length is between 5 and 10 micrometers - have a filamentous structure and are made up of helical protein subunits containing flagellin (a protein). Thanks to these proteins, which differ from bacterium to bacterium by amino acid constitution, flagella represent recognition organs for the human immune system (they constitute the so-called ANTIGEN H).

Three parts can be recognized in each scourge:

  • the filament, which is the protruding portion
  • a hook, through which it attaches to the plasma membrane
  • a basal body, which acts as an anchor to the membrane

The energy necessary to move the flagellum counterclockwise or clockwise is generated inside the basal body. In the first case - considering that the helix formed by the flagellin has a left-handed trend - an active propulsive movement is generated ("swimming", positive chemotaxis), while when the flagellum moves clockwise there is an unproductive movement. movement is influenced by the stimuli picked up by the receptors placed on the surface of the bacterium; if these feel the presence of nutrients, an active propulsive movement is generated; vice versa, if the signal captured is noxious (for example due to the presence of antibacterial substances), there is negative chemotaxis and the bacterium moves away.

The active mobility, conferred to the cell by the presence of flagella, can also favor the penetration of pathogens into the organism.

Pili or Fimbriae

Much smaller than flaggels (they are 0.2 - 2 micrometers in size), they consist of a repetition of protein subunits forming a helical structure. They appear as filamentous appendages, have no function of movement and are more frequent in GRAM negative species, both mobile and immobile.

The proteins that compose them are called piline, while those that characterize the extremities are called adhesins; the latter allow the bacterium to adhere better to surfaces, such as the mucous membranes of the human organism.

Then there are particular types of fimbriae, called FIMBRIE F (F as Fertility), without adhesins and involved in the conjugation process.

Summarizing, therefore, there are pili sexual and pili with adhesive properties.

Bacterial Capsule

The bacterial capsule is a very voluminous envelope consisting essentially of water and mucopolysaccharides, which give it a certain stickiness. It favors the adhesion of the bacterium to certain surfaces or to other bacteria (facilitating the formation of colonies); it also has an important anti-phagocytic and protective function against antibacterial substances, such as lysozyme itself.

The thickness, density and adhesion of the capsule to the cell wall vary from species to species.

Crystalline layer

O layer S; it is made up of proteins and polymers of various kinds, which bind together in an orderly manner. It has a protective function and promotes bacterial aggregation and adhesion to mucous surfaces.


The spore is typical of many bacteria, especially those belonging to the bacillus or clostridium genus. When a bacterial cell enters a phase of metabolic latency due to the lack of suitable conditions for life (lack of nutrients, excessively high or low temperatures, etc.), it surrounds its DNA with a series of protective structures (cortex, mantle and exosporium) and expels it. Thanks to this sort of extremely resistant shell, the spore can survive particularly unfavorable environmental conditions (such as cooking food) and reactivate - with a process called germination - as soon as these become fit for life again.

The process of sporation (ie the formation of the spore) lasts from six to ten hours and is mediated by genetic activations in response to environmental changes; to germinate, however, the spore takes an average of one or two hours.

Other articles on "Accessory Structures of Bacteria"

  1. bacterial cell
  2. bacteria
  3. characteristic bacteria
  4. bacterial toxins
  5. Bacteria: transfer of genetic information
  6. Bacteria: transfer of genetic information
  7. Antibiotics
  8. Categories of antibiotics
  9. Antibiotic resistance
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