Flagella and Fimbria

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Chapter: Pharmaceutical Microbiology : Structure and Function of Bacterial Cells

Flagellum [Pl : Flagella] refers to a thread like structure that provides motility for certain bacte-ria and protozoa (one, few or many per cell) and for spermatazoa (one per cell).


Flagella and Fimbria

 

1. Flagella

 

Flagellum [Pl : Flagella] refers to a thread like structure that provides motility for certain bacte-ria and protozoa (one, few or many per cell) and for spermatazoa (one per cell).

 

It has been observed that the presence of flagella strategically located on certain bacteria (miroorganisms) has been known ever since the beginning of the nineteenth century ; besides, the actual form of flagellation and motility have been exploited judiciously as a taxonomic tool in the logical classification of bacterial variants.

 

Filaments : The ‘flagella’ are nothing but surface appendages invariably found in motile bacte-ria, and appear generally as filaments having diameter ranging between 12–20 nm and length between 6–8 μm. Importantly, the diameter of the individual flagellum in a culture is normally constant ; how-ever, the length may vary accordingly.

 

Location of Flagella : The exact location of the flagella in various bacteria varies widely and specifically ; and could be either polar monotrichous or polar or bipolar or polar peritrichous as shown in Fig. 2.9 ; and the number of flagella per cell also changes with the various bacterial species.

 

Flagellar Apparatus : Basically the flagellar apparatus consists of three distinct parts, namely : (a) filament ; (b) hook ; and (c) basal granule. Importantly, the outermost structural segment of bacteria is the filament which is a fibre essentially comprised of a specific protein termed as flagellin (a subunit having molecular weight 20,000), and this is securedly attached to the basal granule with the help of the hook.

 

Interestingly, both the basal granules and the hook essentially contain certain specific proteins that are antigenically distinct from the flagellin (i.e., the protein of the filament).


 

In fact, the particular structure of the basal body comprises of a small central rod inserted strate-gically into a system of rings as illustrated in Fig. 2.10 below. However, the entire unit just functions fundamentally as a ‘simple motor’. It has been amply demonstrated and established that the meticulous growth of the flagella invariably takes place by the careful addition of the flagellin subunits at the distal end after being drifted through from the cytoplasm, obviously via the hollow core of the very flagellum.


 

Functioning of Flagella : The modus operandi of flagella are as given under :

 

(1) Flagella are fully responsible for the bacterial motility.

 

(2) Deflagellation by mechanical means renders the motile cells immotile.

 

(3) The apparent movement of the bacterial cell usually takes place by the critical rotation of the flagella either in the clockwise or anticlockwise direction along its long axis.

 

(4) Bacterial cell possesses the inherent capacity to alter both the direction of rotation [as in (3) above] and the speed ; besides, the meticulous adjustment of frequency of ‘stops’ and ‘starts’ by the appropriate movement of the flagella.

 

(5) Evidently, the flagellated peritrichal* bacteria usually swim in a straight line over moderate distances. In actual practice, these swim-across straight line runs are interrupted frequently by abrupt alterations in the direction that ultimately leads to tumbling. Therefore, the move-ment of the bacteria is believed to be zig-zag.

 

(6) It has been observed that the phenomenon of smooth swimming in a fixed direction is invari-ably mediated by the rotation of flagella in an anticlockwise direction ; whereas, the process of tumbling in a zig-zag direction is usually caused by the rotation of flagella in a clockwise direction.

 

(7) The presence of ‘polar flagella’ in bacteria affords a distinct change in the direction that usually takes place by the reciprocal alteration in the direction of rotation.

 

2. Fimbriae [or Pili*]

 

Fimbriae or Pili are hollow, non-helical, filamentous hair-like structures that are apparently thinner, shorter, and more numerous than flagella. However, these structures do appear on the surface of the only Gram negative bacteria and are virtually distinct from the flagella.

 

Another school of thought rightly differentiates the terminology ‘fimbriae’ exclusively reserved for all hair-like structures ; whereas, other structures that are directly and intimately involved in the actual transfer of genetic material solely are termed as ‘pili’. Likewise, the bacterial flagella that may be visualized conveniently with the help of a light microscope after only suitable staining ; and the bacterial pili can be seen vividly only with the aid of an electron microscope.

 

Salient Features of Fimbriae : Some of the important salient features of ‘fimbriae’ are as enumerated under :

 

(1) At least 5 to 6 fimbriae variants have been recognized besides the sex pili.

 

(2) Type I fimbriae has been characterized completely.

 

(3) They contain a particular protein known as pilin having molecular weight of 17,000 daltons.

 

(4) The fimbriae are found to be spread over the entire cell surface. These have a diameter of 7 nm and a length ranging between 0.5 to 2 μm ; besides, an empty core of 2 to 2.5 nm.

 

(5) The pilin subunits are duly arranged in a helical manner having the pitch of the helix** almost nearly at 2.3 μm.

 

(6) In addition to the Type-I fimbriae, the Gram-negative bacteria invariably own a special category of pili termed as the sex pili (or F-pili), the synthesis of which is predominently directed by the sex factor (or F-factor). It has been observed that the sex pili do have a uniform diameter of approximately 9 nm, and a length almost nearing between 1-20 μm.

 

(7) Very much akin to the flagella, both fimbriae and pili are observed to originate from the basal bodies strategically located within the cytoplasm. Interestingly, neither fimbriae nor pili seem to be essential for the survival of the bacteria.

 

Human Infection : It has been demonstrated that certain pili do play a major role in causing and spreading human infection to an appreciable extent by permitting the pathogenic bacteria to get strategically attached to various epithelial cells lining the genito urinary, intestinal, or respiratory tracts specifically. It is worthwhile to mention here that this particular attachment exclusively checks and pre-vents the bacteria from being washed away critically by the incessent flow of either mucous or body fluids thereby allowing the infection to be established rather firmly.


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